Add ThreadLeakLingering option to Rest client tests

Some Rest tests use the Sun HTTP server which has lingering threads after shutdown. Similar to ESTestCase, this adds an
option to wait 5 seconds for these threads to terminate.

client/test/src/main/java/org/elasticsearch/client/RestClientTestCase.java

@@ -26,6 +26,7 @@ import com.carrotsearch.randomizedtesting.annotations.SeedDecorators;
 import com.carrotsearch.randomizedtesting.annotations.TestMethodProviders;
 import com.carrotsearch.randomizedtesting.annotations.ThreadLeakAction;
 import com.carrotsearch.randomizedtesting.annotations.ThreadLeakGroup;
+import com.carrotsearch.randomizedtesting.annotations.ThreadLeakLingering;
 import com.carrotsearch.randomizedtesting.annotations.ThreadLeakScope;
 import com.carrotsearch.randomizedtesting.annotations.ThreadLeakZombies;
 import com.carrotsearch.randomizedtesting.annotations.TimeoutSuite;
@@ -38,7 +39,8 @@ import com.carrotsearch.randomizedtesting.annotations.TimeoutSuite;
 @ThreadLeakGroup(ThreadLeakGroup.Group.MAIN)
 @ThreadLeakAction({ThreadLeakAction.Action.WARN, ThreadLeakAction.Action.INTERRUPT})
 @ThreadLeakZombies(ThreadLeakZombies.Consequence.IGNORE_REMAINING_TESTS)
+@ThreadLeakLingering(linger = 5000) // 5 sec lingering
 @TimeoutSuite(millis = 2 * 60 * 60 * 1000)
 public abstract class RestClientTestCase extends RandomizedTest {
 
-}
+}