Before breaking the connection we have to check that there's no query
  executing at the moment. Otherwise it can lead to crash in embedded server.


client/mysqltest.c:
  Bug #33334 mysqltest_embedded crashes when disconnecting before reap.
  
  Wait until the query thread is finished before we break the connection.
  Waiting part moved to a separate wait_query_thread_end() function
mysql-test/r/flush.result:
  Bug #33334 mysqltest_embedded crashes when disconnecting before reap.
  
  test result
mysql-test/t/flush.test:
  Bug #33334 mysqltest_embedded crashes when disconnecting before reap.
  
  test case