ERROR MESSAGE

Description:- Mtr test, "main.mysqldump" is failing with an
assert when "mysqlimport" client utility is executed with
the option "--use_threads".

Analysis:- "mysqlimport" uses the option, "--use_threads",
to spawn worker threads to complete its job in parallel. But
currently the main thread is not waiting for the worker
threads to complete its cleanup, rather just wait for the
worker threads to say its done doing its job. So the cleanup
is done in a race between the worker threads and the main
thread. This lead to an assertion failure.

Fix:- "my_thread_join()" is introduced in the main thread to
join all the worker threads it have spawned. This will let
the main thread to wait for all the worker threads to
complete its cleanup before calling "my_end()".