test: evaluate if this prevents the data loss that causes no alerts being loaded.

Original commit: elastic/x-pack-elasticsearch@611f3df816

src/test/java/org/elasticsearch/alerts/NoMasterNodeTests.java

@@ -112,6 +112,7 @@ public class NoMasterNodeTests extends AbstractAlertingTests {
                     .setAlertSource(alertSource)
                     .get();
         }
+        ensureGreen();
 
         for (int i = 1; i <= numberOfFailures; i++) {
             logger.info("Failure round {}", i);