[ML] Use consistent assertBusy() maxWaitTime in datafeed test (#138202)

Other uses of assertBusy() when checking datafeed stats in
DatafeedJobsIT.testDatafeedTimingStats_DatafeedRecreated() use a
maxWaitTime of 30 seconds instead of the default of 10, which is
necessary to account for possible delays in persisting stat values.

Update the maxWaitTime on the first datafeed stats assert to 30 seconds

Closes #137207

(cherry picked from commit f236373)

Author: DonalEvans

x-pack/plugin/ml/qa/native-multi-node-tests/src/javaRestTest/java/org/elasticsearch/xpack/ml/integration/DatafeedJobsIT.java

@@ -250,7 +250,7 @@ public void testDatafeedTimingStats_DatafeedRecreated() throws Exception {
 
             putDatafeed(datafeedConfig);
             // Datafeed did not do anything yet, hence search_count is equal to 0.
-            assertBusy(() -> assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), equalTo(0L)));
+            assertBusy(() -> assertDatafeedStats(datafeedId, DatafeedState.STOPPED, job.getId(), equalTo(0L)), 30, TimeUnit.SECONDS);
             startDatafeed(datafeedId, 0L, now.toEpochMilli());
 
             // First, wait for data processing to complete