Return test annotation

This commit returns a test annotation that is needed on
PreBuiltXPackTransportClientTests because the test derives from
RandomizedTest and not ESTestCase and its ilk.

Original commit: elastic/x-pack-elasticsearch@1badced090

elasticsearch-transport-client/src/test/java/org/elasticsearch/xpack/client/PreBuiltXPackTransportClientTests.java

@@ -20,6 +20,7 @@ import static org.junit.Assert.assertEquals;
  */
 public class PreBuiltXPackTransportClientTests extends RandomizedTest {
 
+    @Test
     public void testPluginInstalled() {
         try (TransportClient client = new PreBuiltXPackTransportClient(Settings.EMPTY)) {
             Settings settings = client.settings();