refactor(test): fix test failure due to NullPointerException during u…

…pgrade of groovy 4.x (#1444)

While upgrading groovy to 4.0.15, encountered the errors in echo-test module, similar to following error is appearing for multiple tests:
```
Failed to instantiate spec 'PipelineCacheSpec'
org.spockframework.util.InternalSpockError: Failed to instantiate spec 'PipelineCacheSpec'
	at app//org.spockframework.runtime.PlatformSpecRunner.createSpecInstance(PlatformSpecRunner.java:80)
	at app//org.spockframework.runtime.PlatformSpecRunner.runSharedSpec(PlatformSpecRunner.java:45)
	at app//org.spockframework.runtime.SpecNode.prepare(SpecNode.java:32)
	at app//org.spockframework.runtime.SpecNode.prepare(SpecNode.java:11)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$prepare$2(NodeTestTask.java:123)
	at app//org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.prepare(NodeTestTask.java:123)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:90)
	at [email protected]/java.util.ArrayList.forEach(ArrayList.java:1511)
	at app//org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:41)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$6(NodeTestTask.java:155)
	at app//org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:141)
	at app//org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$9(NodeTestTask.java:139)
	at app//org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:138)
	at app//org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:95)
	at app//org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:35)
	at app//org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
	at app//org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:54)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:107)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)
	at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:114)
	at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:86)
	at org.junit.platform.launcher.core.DefaultLauncherSession$DelegatingLauncher.execute(DefaultLauncherSession.java:86)
	at org.junit.platform.launcher.core.SessionPerRequestLauncher.execute(SessionPerRequestLauncher.java:53)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.processAllTestClasses(JUnitPlatformTestClassProcessor.java:99)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor$CollectAllTestClassesExecutor.access$000(JUnitPlatformTestClassProcessor.java:79)
	at org.gradle.api.internal.tasks.testing.junitplatform.JUnitPlatformTestClassProcessor.stop(JUnitPlatformTestClassProcessor.java:75)
	at org.gradle.api.internal.tasks.testing.SuiteTestClassProcessor.stop(SuiteTestClassProcessor.java:62)
	at [email protected]/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at [email protected]/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
	at [email protected]/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at [email protected]/java.lang.reflect.Method.invoke(Method.java:568)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:36)
	at org.gradle.internal.dispatch.ReflectionDispatch.dispatch(ReflectionDispatch.java:24)
	at org.gradle.internal.dispatch.ContextClassLoaderDispatch.dispatch(ContextClassLoaderDispatch.java:33)
	at org.gradle.internal.dispatch.ProxyDispatchAdapter$DispatchingInvocationHandler.invoke(ProxyDispatchAdapter.java:94)
	at jdk.proxy2/jdk.proxy2.$Proxy5.stop(Unknown Source)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker$3.run(TestWorker.java:193)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.executeAndMaintainThreadName(TestWorker.java:129)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:100)
	at org.gradle.api.internal.tasks.testing.worker.TestWorker.execute(TestWorker.java:60)
	at org.gradle.process.internal.worker.child.ActionExecutionWorker.execute(ActionExecutionWorker.java:56)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:113)
	at org.gradle.process.internal.worker.child.SystemApplicationClassLoaderWorker.call(SystemApplicationClassLoaderWorker.java:65)
	at app//worker.org.gradle.process.internal.worker.GradleWorkerMain.run(GradleWorkerMain.java:69)
	at app//worker.org.gradle.process.internal.worker.GradleWorkerMain.main(GradleWorkerMain.java:74)
Caused by: java.lang.NullPointerException: Cannot invoke method withRunAsUser() on null object
	at org.codehaus.groovy.runtime.NullObject.invokeMethod(NullObject.java:98)
	at org.codehaus.groovy.vmplugin.v8.IndyGuardsFiltersAndSignatures.invokeGroovyObjectInvoker(IndyGuardsFiltersAndSignatures.java:151)
	at org.codehaus.groovy.vmplugin.v8.IndyInterface.fromCache(IndyInterface.java:321)
	at com.netflix.spinnaker.echo.test.RetrofitStubs$Trait$Helper.$init$com_netflix_spinnaker_echo_test_RetrofitStubs__enabledJenkinsTriggerWithRunAsUser(RetrofitStubs.groovy:28)
	at org.codehaus.groovy.vmplugin.v8.IndyInterface.fromCache(IndyInterface.java:321)
	at com.netflix.spinnaker.echo.pipelinetriggers.PipelineCacheSpec.<init>(PipelineCacheSpec.groovy)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
	at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:77)
	at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
	at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:499)
	at java.base/java.lang.reflect.ReflectAccess.newInstance(ReflectAccess.java:128)
	at java.base/jdk.internal.reflect.ReflectionFactory.newInstance(ReflectionFactory.java:347)
	at java.base/java.lang.Class.newInstance(Class.java:645)
	at org.spockframework.runtime.PlatformSpecRunner.createSpecInstance(PlatformSpecRunner.java:78)
	... 51 more
```
To fix this issue, refactored the code to replace the reference `enabledJenkinsTrigger` with a new object using builder pattern of `Trigger` class.

echo-test/src/main/groovy/com/netflix/spinnaker/echo/test/RetrofitStubs.groovy

@@ -25,7 +25,7 @@ trait RetrofitStubs {
   final String url = "http://echo"
   final Trigger enabledJenkinsTrigger = Trigger.builder().enabled(true).type('jenkins').master('master').job('job').build()
   final Trigger disabledJenkinsTrigger = Trigger.builder().enabled(false).type('jenkins').master('master').job('job').build()
-  final Trigger enabledJenkinsTriggerWithRunAsUser = enabledJenkinsTrigger.withRunAsUser("user@managed-service-account")
+  final Trigger enabledJenkinsTriggerWithRunAsUser = Trigger.builder().enabled(true).type('jenkins').master('master').job('job').build().withRunAsUser("user@managed-service-account")
   final Trigger enabledTravisTrigger = Trigger.builder().enabled(true).type('travis').master('master').job('job').build()
   final Trigger disabledTravisTrigger = Trigger.builder().enabled(false).type('travis').master('master').job('job').build()
   final Trigger enabledWerckerTrigger = Trigger.builder().enabled(true).type('wercker').master('master').job('job').build()