otherwise, e.g.

  ./mtr main.mysql_install_db_win_admin,innodb

results in sporadic

mysql-test-run: *** ERROR: Could not run main.mysql_install_db_win_admin with 'innodb' combination(s)

depending on whether it'll process the skip (not windows admin)
or the innodb.combinations first (if skip is processed first,
innodb combination wasn't, making the further code think that the test
doesn't have innodb combination)