Mark CSIT#testLoggerLevelUpdate as awaits fix

This commit marks ClusterSettingsIT#testLoggerLevelUpdate as awaiting a
fix due to a test bug.

core/src/test/java/org/elasticsearch/cluster/settings/ClusterSettingsIT.java

@@ -330,6 +330,7 @@ public class ClusterSettingsIT extends ESIntegTestCase {
         }
     }
 
+    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/20318")
     public void testLoggerLevelUpdate() {
         assertAcked(prepareCreate("test"));
         final IllegalArgumentException e =