[ML] Increase datafeed integration test timeout for slow machines (#39311)

The assertBusy() that waits the default 10 seconds for a
datafeed to complete very occasionally times out on slow
machines.  This commit increases the timeout to 60 seconds.
It will almost never actually take this long, but it's
better to have a timeout that will prevent time being
wasted looking at spurious test failures.

Author: droberts195

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

@@ -1106,7 +1106,7 @@ private void startDatafeedAndWaitUntilStopped(String datafeedId, String authHead
             } catch (Exception e) {
                 throw new RuntimeException(e);
             }
-        });
+        }, 60, TimeUnit.SECONDS);
     }
 
     private void waitUntilJobIsClosed(String jobId) throws Exception {