[Test] Fix slow rollup job task test

Incorrect latch caused this test to run slowly (until the await
finished), and could probably cause failure due to incorrect ordering

Original commit: elastic/x-pack-elasticsearch@ebeb8655da

plugin/rollup/src/test/java/org/elasticsearch/xpack/rollup/job/RollupJobTaskTests.java

@@ -590,7 +590,7 @@ public class RollupJobTaskTests extends ESTestCase {
             @Override
             public void onResponse(StopRollupJobAction.Response response) {
                 assertTrue(response.isStopped());
-                latch.countDown();
+                latch2.countDown();
             }
 
             @Override