test: after first elected master node has been stopped assert that watcher is started and only running once.

Original commit: elastic/x-pack-elasticsearch@e193b80215

src/test/java/org/elasticsearch/watcher/test/integration/NoMasterNodeTests.java

@@ -155,6 +155,8 @@ public class NoMasterNodeTests extends AbstractWatcherIntegrationTests {
 
         // We still have 2 master node, we should recover from this failure:
         internalTestCluster().stopCurrentMasterNode();
+        ensureWatcherStarted(false);
+        ensureWatcherOnlyRunningOnce();
         assertWatchWithMinimumPerformedActionsCount("_watch_id", 2, false);
 
         // Stop the elected master, no new master will be elected b/c of m_m_n is set to 2