Unpin minitest to permit later 5.x
minitest was pinned to 5.1.x during #7230, the upgrade to Ruby on Rails 4.1. The exact reason is unclear (https://github.com/theforeman/foreman/pull/2870#discussion-diff-43727271).
It can be upgraded, but may require some changes to incorrect tests and fixes for deprecation warnings issued for Minitest 6 (e.g.
assert_equal nil, [..] should be