[ticket/12963] Fix the migration's finder for the tests (path, new instance)

PHPBB3-12963
Merge remote-tracking branch 'Nicofuma/ticket/12963' into develop-ascraeus

* Nicofuma/ticket/12963:
  [ticket/12963] Fix the migration's finder for the tests (path, new instance)
Merge branch 'develop-ascraeus' into develop

* develop-ascraeus:
  [ticket/12963] Fix the migration's finder for the tests (path, new instance)