Get rid of unnecessary gem "minitest" calls

[amatsuda]

This patch simply removes unnecessary gem activations of minitest.

Motivation / Background

minitest used to be a CRuby's stdlib from Rubies >= 1.9.1 to < 2.2.0.
And the gem 'minitest' call in the Rails codebase was added at 6f74d36 because at that time require 'minitest' could load either a gem version or the stdlib one, and we wanted to make sure not to load the stdlib minitest.

2.years.since then, CRuby's bundled minitest had been removed in 2.2.0 at ruby/ruby@7cda822
and now that Ruby < 2.2 is no longer supported in Rails 7, we can just safely drop this gem "minitest" call in favor of simple require that is already written there.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]