Fix tests

Author: dandavison

tests/nexus/test_handler.py

@@ -343,6 +343,7 @@ class UnsuccessfulResponse:
 
 class _TestCase:
     operation: str
+    service_defn: str = "MyService"
     input: Input = Input("")
     headers: dict[str, str] = {}
     expected: SuccessfulResponse
@@ -621,8 +622,8 @@ class OperationError(_FailureTestCase):
 
 
 class UnknownService(_FailureTestCase):
-    operation = "echo"
-    service = "NonExistentService"
+    service_defn = "NonExistentService"
+    operation = "<should not be used by test>"
     expected = UnsuccessfulResponse(
         status_code=404,
         retryable_header=False,
@@ -710,7 +711,9 @@ async def _test_start_operation(
         server_address=f"http://127.0.0.1:{env._http_port}",  # type: ignore
         endpoint=endpoint,
         service=(
-            MyService.__name__ if with_service_definition else MyServiceHandler.__name__
+            test_case.service_defn
+            if with_service_definition
+            else MyServiceHandler.__name__
         ),
     )