Wait for events in waitForRelocation

Adds a `waitForEvents(Priority.LANGUID)` to the cluster health request in
`ESIntegTestCase#waitForRelocation()` to deal with the case that this health
request returns successfully despite the fact that there is a pending reroute task which
will relocate another shard.

Relates elastic#44433
Fixes elastic#45003

test/framework/src/main/java/org/elasticsearch/test/ESIntegTestCase.java

@@ -900,7 +900,7 @@ public ClusterHealthStatus waitForRelocation() {
      * using the cluster health API.
      */
     public ClusterHealthStatus waitForRelocation(ClusterHealthStatus status) {
-        ClusterHealthRequest request = Requests.clusterHealthRequest().waitForNoRelocatingShards(true);
+        ClusterHealthRequest request = Requests.clusterHealthRequest().waitForNoRelocatingShards(true).waitForEvents(Priority.LANGUID);
         if (status != null) {
             request.waitForStatus(status);
         }