Add minitest to Gemfile. Fix #3826

There are occasions where minitest is not bundled in packaged
versions of ruby that some OSes distribute (eg Fedora). Adding
minitest to Gemfile ensures that it gets loaded and tests can run.

Signed-off-by: Axilleas Pipinellis <axilleas@archlinux.gr>
Merge pull request #3852 from axilleas/minitest-fix

Add minitest to Gemfile. Fix #3826