Tests: Ensure watcher is stopped before restarting

The EmailSecretsIntegrationTests did not properly wait for watcher to be
stopped before starting it again.

This test failed a few times per week across master and 6.x branches.

Original commit: elastic/x-pack-elasticsearch@cf20d58b0b

plugin/watcher/src/test/java/org/elasticsearch/xpack/watcher/notification/email/EmailSecretsIntegrationTests.java

@@ -123,6 +123,7 @@ public class EmailSecretsIntegrationTests extends AbstractWatcherIntegrationTest
 
         // now we restart, to make sure the watches and their secrets are reloaded from the index properly
         assertAcked(watcherClient.prepareWatchService().stop().get());
+        ensureWatcherStopped();
         assertAcked(watcherClient.prepareWatchService().start().get());
         ensureWatcherStarted();