[MINOR UPDATE]: Fix Hadoop 3 impersonation test errors during CI runs (…

…apache#2822)

* Shut down MiniDFS cluster in TestInboundImpersonation.selectChainedView.

* Ensure that MiniDFS is using simple auth in BaseTestImpersonation.

exec/java-exec/src/test/java/org/apache/drill/exec/impersonation/BaseTestImpersonation.java

@@ -103,6 +103,13 @@ protected static void startMiniDfsCluster(String testClass, boolean isImpersonat
       // Set the proxyuser settings so that the user who is running the Drillbits/MiniDfs can impersonate other users.
       dfsConf.set(String.format("hadoop.proxyuser.%s.hosts", processUser), "*");
       dfsConf.set(String.format("hadoop.proxyuser.%s.groups", processUser), "*");
+
+      // It isn't clear exactly when or why the following reinitialisation is
+      // needed, but without it the test subclasses of this class may crash
+      // when run in the GitHub CI with Mini DFS stating "Running in secure
+      // mode, but config doesn't have a keytab".
+      dfsConf.set("hadoop.security.authentication", "simple");
+      UserGroupInformation.setConfiguration(dfsConf);
     }
 
     // Start the MiniDfs cluster

exec/java-exec/src/test/java/org/apache/drill/exec/impersonation/TestInboundImpersonation.java

@@ -92,6 +92,8 @@ public void selectChainedView() throws Exception {
             .go();
       }
       adminClient.resetSystem(ExecConstants.IMPERSONATION_POLICIES_KEY);
+    } finally {
+      stopMiniDfsCluster();
     }
   }