Wait for task to start in reindex test

`RethrottleTests#testReindex` fail in CI:
https://elasticsearch-ci.elastic.co/job/elastic+elasticsearch+master+multijob-intake/1274/console

I was unable to reproduce it locally but it *looks* like a race to start
the task. So I've added a wait for it to start just in case.

modules/reindex/src/test/java/org/elasticsearch/index/reindex/RethrottleTests.java

@@ -57,6 +57,9 @@ public class RethrottleTests extends ReindexTestCase {
         request.source().setSize(1);             // Make sure we use multiple batches
         ListenableActionFuture<? extends BulkIndexByScrollResponse> responseListener = request.execute();
 
+        // Wait for the task to start
+        assertBusy(() -> assertEquals(1, client().admin().cluster().prepareListTasks().setActions(actionName).get().getTasks().size()));
+
         // Now rethrottle it so it'll finish
         ListTasksResponse rethrottleResponse = rethrottle().setActions(actionName).setRequestsPerSecond(Float.POSITIVE_INFINITY).get();
         assertThat(rethrottleResponse.getTasks(), hasSize(1));