Make the RuboCop::Minitest::Test suite re-usable by gem consumers

[DougEdey]

I've been writing custom cops for various jobs for a while now, but I've never had a consistent way to write tests for products that use minitest as the test suite (officially Rubocop only provides rspec helpers) , I started to look around to see what this Gem did and figured it would make sense to make rubocop-minitest provide a minitest class for others that write custom cops for their own code bases.

This does a couple of things

1. Makes a new Rubocop::Minitest::Test class that includes the code from assert_offense (I don't feel like it needs to be a module), and it also uses declarative testing for people to use.
2. I moved the "real" test suites in this gem to use the new base class, and kept the code blocks being tested as Minitest::Test
3. I added declarative testing since all the test suites I've been in have it, where instead of using def test_something_does_something...end you can do test "something does something" do...end which makes it easier to read test names.

I tested this locally in an app of mine and it works great! So it can allow a consistent way for people to write custom cops and easily test them!

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Before submitting the PR make sure the following are checked:

• [x] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• [x] Wrote good commit messages.
• [-] Commit message starts with [Fix #issue-number] (if the related issue exists).
• [x] Feature branch is up-to-date with master (if not - rebase it).
• [x] Squashed related commits together.
• [x] Added tests.
• [x] Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• [x] 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.

[exterm]

This makes a lot of sense to me and I'd love to use it.

[Earlopain]

If this is being exposed like this then I think it needs some documetation on how to pass global and cop specific config to a test.

See https://github.com/rubocop/rubocop-minitest/blob/b446022ea09b3f5558df9c0106c9e714c6fc1ec5/test/rubocop/cop/minitest/global_expectations_test.rb#L504-L513 and https://github.com/rubocop/rubocop-minitest/blob/b446022ea09b3f5558df9c0106c9e714c6fc1ec5/test/rubocop/cop/minitest/multiple_assertions_test.rb#L339-L343

[DougEdey]

@Earlopain Updated the casing of RuboCop, updated the sample docs (both inline and README.md), handling the nil case for @cop

[DougEdey]

Squashed commits and confirmed CI is passing locally

[DougEdey]

@koic 👋 Could I ask for a review?