Increase Timeout for Waiting on Tasks in REST Tests (#64707) (#64718)

10s is pretty tight here considering that some x-pack related tests
might have a running create-index tasks in the backgroud after a test
that require multiple CS updates and can randomly take a few seconds on
slow IO etc.

closes #64580

test/framework/src/main/java/org/elasticsearch/test/rest/ESRestTestCase.java

@@ -421,7 +421,7 @@ public abstract class ESRestTestCase extends ESTestCase {
             } catch (final IOException e) {
                 throw new AssertionError("error getting active tasks list", e);
             }
-        });
+        }, 30L, TimeUnit.SECONDS);
     }
 
     /**