Removing minitest from the katello rpm spec

Given that we run unit tests under travis after every commit,
running em again while building rpm feel redundant.
Also we were using minitest only as "source code linting" tool.
Rubocop will catch many more of those in travis.
So no point for having minitest in rpm spec and having to maintain it.
Removing them from the spec

Also contains rubo cop changes.
Merge pull request #2834 from parthaa/minus-minitest

Removing minitest from the katello rpm spec