Increase time out tolerance in NoMasterNodeIT.

see https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-intake/746/console

core/src/test/java/org/elasticsearch/cluster/NoMasterNodeIT.java

@@ -184,7 +184,7 @@ public class NoMasterNodeIT extends ESIntegTestCase {
                 assertThat(e.status(), equalTo(RestStatus.SERVICE_UNAVAILABLE));
             } else {
                 // timeout is 5000
-                assertThat(System.currentTimeMillis() - now, lessThan(timeout.millis() + 50));
+                assertThat(System.currentTimeMillis() - now, lessThan(timeout.millis() + 300));
             }
         }
     }