If the index request is executed before the mapping update is applied on
the IndexShard, the index request will perform a dynamic mapping update.
This mapping update will be timeout (i.e, ProcessClusterEventTimeoutException)
because the latch is not open. This leads to the failure of the index
request and the test. This commit makes sure the mapping is ready
before we execute the index request.
Closes#37807
557fcf915e