behave randomly with mysql_change_user.
  
The problem was that global status variables were not updated
in THD::check_user(), so thread statistics were lost after
COM_CHANGE_USER.
  
The fix is to update global status variables with the thread ones
before preparing the thread for new user.


mysql-test/r/change_user.result:
  Update result file.
mysql-test/t/change_user.test:
  Add a test case for Bug#31222: com_% global status counters
  behave randomly with mysql_change_user.
sql/sql_class.cc:
  Update global status variables when we're handling
  COM_CHANGE_USER for a thread.