Add Markdown annotation idempotency test

[aouxwoux]

Adds a regression test for re-running AnnotateRb on an already annotated Markdown model. It verifies the behavior merged in #354 without duplicating that implementation.

Validation:

• bundle exec rake spec:unit
• bundle exec standardrb spec/lib/annotate_rb/model_annotator/single_file_annotator_spec.rb
• git diff --check

[drwl]

Thanks for submitting this PR. It seems like #354 covers the same changes and also includes a more in depth write up, so I went and merged that one.

I'd hate to completely dismiss a contribution, but I did notice that the single_file_annotator_spec.rb change could still be valuable. If you want to submit that I'd be happy to merge that in.

[OdenTakashi]

Thanks for adding this. Looks good to me.