feat: support correcting assert_nil and refute_nil to RSpec/Rails/MinitestAssertions

[G-Rath]

I'm not sure how far folks want to take this (if at all), but I got the impression from rubocop/rubocop-rspec_rails#7 & #1517 that it was desirable to support at least some more assertions, and this felt like a pretty straightforward one to land.

I've deliberately gone the "boring" route of just effectively duplicating everything twice rather than try to use a single set of methods because lines of code are cheap and if more matchers are supported there might be better ways to deduplicate that are not compatible with one or both of these matchers.

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop is configured as Enabled: true in .rubocop.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded: "<<next>>" in default/config.yml.

If you have modified an existing cop's configuration options:

• Set VersionChanged: "<<next>>" in config/default.yml.

[G-Rath]

Initially I thought it would be best to correct to be_nil since that's what other cops already do for to eq(nil) but I realised that's why it actually shouldn't because Rubocop will automatically apply that fix on the same pass and then people who have turned off the default cops for eq(nil) -> be(nil) -> be_nil will end up with their preferred syntax too.

[bquorning]

Looks good!

Perhaps consider wrapping specs in two context blocks, one for the “equal” assertions and one for the “nil” assertions.

[G-Rath]

oh yeah good point! I've been trailing minitest recently and so had completely forgotten we've got context here 🙈

[bquorning]

@pirj, could I get a second opinion on this PR, please?

[pirj]

Fantastic, thank you!

[pirj]

It was a miss in the original implementation, it’s confusing that a is “expected, and b is “actual”.
Up to you if you want to fix this along the way or leave out.

[G-Rath]

Good point - I'll leave it for now since this diff is already pretty big, but happy to do a follow up addressing it

[pirj]

Thanks again!

[ydah]

This change should not have been made. This should only be changed when the default settings are changed. For example, when you add a configurable option or change the default for a configurable option.

[ydah]

fix: #1777

[bquorning]

Oops, my bad. I should have caught that in review.

Thanks for discovering, and fixing.

[G-Rath]

for what it's worth I feel like that was required by bundle exec rake, but it might have been I just messed something up or changed something else I shouldn't have locally to make it think this needed updating 🤷

I'll keep this in mind for future contributions and let you know if I do find the tasks behaving wrongly

[ydah]

Never mind! Thank you for your work.