Fixes #21718 - Test

fixes #19131 - upgrade minitest to latest 5.x

- override minitest's `_` expectation method with the gettext method
- rename `location` let helper, conflicted with a minitest method name
- fix `assert_equal nil, [..]` deprecation warnings, prefer `assert nil`...

fixes #16557 - move tests into modern Rails layout

Models, controllers, mailers and helpers are now in test/ subdirs
matching app/ subdirs. Service, miscellaneous and old test/lib/ tests
are now in test/unit/ separated from models, which obsoletes the custom...