mysql-test/mysql-test-run.pl:
  Add a way to remove options which mysql-test-run.pl no longer uses.  Since
  mysql-test-run.pl passes unrecognized options through to mysqld directly,
  it is not possible to just remove an option.  Otherwise it would be given
  to mysqld, which probably will not recognize it and will fail to start.
  
  Instead, we now explicitly ignore the option, and print a warning to
  the user.