Handle assertions in conditionals branches in Minitest/MultipleAssertions cop

[fatkodima]

These changes are to allow to have assertions like:

def test_foo
  if ActiveRecord::VERSION >= "7.0"
    assert ActiveRecord.old_foo_method(1, 2)
  else
    assert ActiveRecord.new_foo_method(1, 2)
  end 
end

A recent example that inspired this - https://github.com/ixti/sidekiq-throttled/pull/142/files#diff-87c291ea2af263111982285d3dea3083faa62260cc6080cc5396f4bf82a37cf4R22-R27 (it is using rspec, but it does not matter)

[koic]

I worry about losing the symmetry between #bad and #good examples. Can you replace the added example code with a description of the cop using natural language?

[koic]

Same for case_match?

[koic]

Same for numblock?

[koic]

Can you add a source code comment to the line which line is being counted?

[koic]

Ditto.

[fatkodima]

Applied suggested changes. CI is failing, because rubocop now requires ruby 2.7, but this gem - ruby 2.6. So it is needed to first drop support for ruby 2.6 and then this PR can be updated.

Should I open a separate PR with that change or should you?

[koic]

Should I open a separate PR with that change or should you?

The build error has been fixed by #250. Either way, I understand that the issue is unrelated to this PR change. Thank you :-)

[fatkodima]

Updated, now CI is green 🟢