Mark monitoring bwc indices tests awaitsfix

They have grown a bit unstable. I'll work to stabilize them
in the morning.

Original commit: elastic/x-pack-elasticsearch@e5f2fdcfb7

elasticsearch/src/test/java/org/elasticsearch/xpack/monitoring/OldMonitoringIndicesBackwardsCompatibilityIT.java

@@ -78,6 +78,12 @@ public class OldMonitoringIndicesBackwardsCompatibilityIT extends AbstractOldXPa
         settings.putProperties(httpExporter, k -> true, k -> MonitoringSettings.EXPORTERS_SETTINGS.getKey() + "my_exporter." + k);
     }
 
+    @AwaitsFix(bugUrl="waiting until stable")
+    @Override
+    public void testOldIndexes() throws Exception {
+        super.testOldIndexes();
+    }
+
     @Override
     protected void checkVersion(Version version) throws Exception {
         if (version.before(Version.V_2_3_0)) {