- Unify on MTR_SKIP_TEST_LIST in both d/rules and autopkgtests
- Unify MTR command in both d/rules and autopkgtests
- Make d/rules section more verbose to help debugging why tests
  sometimes ran and sometimes not
- If MTR fails, make the log a bit more verbose
  (inspired by https://github.com/MariaDB/buildbot/pull/76/files)