[TEST] Provides generics for action in ESUsersRealmTests

Although the build passes on the command line the Eclipse compiler complains that the client.execute() call on line 216 does not have correct arguments because of the lack of generics. This changes adds the generics to the action variable to solve the error in Eclipse. The change is very low risk and should not adversely affect the build on the command line nor in intelliJ IDEA

Original commit: elastic/x-pack-elasticsearch@08f3548556

shield/src/test/java/org/elasticsearch/shield/authc/esusers/ESUsersRealmTests.java

@@ -196,7 +196,7 @@ public class ESUsersRealmTests extends ESTestCase {
             }
         };
         RestRequest restRequest = mock(RestRequest.class);
-        final Action action = mock(Action.class);
+        final Action<ActionRequest, ?, ?> action = mock(Action.class);
         final ActionListener listener = mock(ActionListener.class);
         BaseRestHandler handler = new BaseRestHandler(Settings.EMPTY, restController, client) {
             @Override