[ticket/12963] Correctly load migration files from migration/ in tests as well

PHPBB3-12963
Merge pull request #2964 from nickvergessen/ticket/12963

[ticket/12963] Correctly load migration files from migration/ in tests as well

* nickvergessen/ticket/12963:
  [ticket/12963] Correctly load migration files from migration/ in tests as well
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12963] Correctly load migration files from migration/ in tests as well