Add new Minitest/RedundantMessageArgument cop

[koic]

This PR adds new Minitest/RedundantMessageArgument cop that detects redundant message argument in assertion methods. The message argument nil is redundant because it is the default value.

# bad
assert_equal(expected, actual, nil)

# good
assert_equal(expected, actual)
assert_equal(expected, actual, 'message')

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[andyw8]

I'm curious about the context for this, where did you encounter it?

[koic]

zdennis/activerecord-import@85cbdea

[andyw8]

I see. I'm wondering if it might be better for this to be in Minitest, e.g. it can raise if nil is given for that argument?

[koic]

That's an interesting idea. If you're up for it, please propose it to Minitest.
Regardless, raise operates at runtime, while this cop is for static analysis, so the detection timing differs. It shouldn't detract from the value of this cop, I think.