CI: Treat mdtest Markdown files as code

[sharkdp]

Summary

Make sure we run the tests for mdtest-only changes.

Test Plan

Tested if positive glob patterns override negative patterns here: https://codepen.io/mrmlnc/pen/OXQjMe

Will test using the changes in #14652 (where tests were previously skipped)

[AlexWaygood]

LGTM if it works. Is it worth pushing a temporary commit to an md file here to make sure it triggers CI, and then reverting it again afterwards?

[AlexWaygood]

Oh I guess CI will always be triggered here anyway, because this PR edits the workflow file. So the temporary commit would also have to remove the line in this workflow file that means that CI is triggered if the workflow file itself is edited in a PR

[github-actions]

ruff-ecosystem results

Linter (stable)

✅ ecosystem check detected no linter changes.

Linter (preview)

✅ ecosystem check detected no linter changes.

Formatter (stable)

✅ ecosystem check detected no format changes.

Formatter (preview)

✅ ecosystem check detected no format changes.

[sharkdp]

I would have to be careful to remove .github/workflows/ci.yml everywhere and add a new !.github/workflows/ci.yml pattern in the code section. And then I might still miss something and we still don't know if CI ran because of the mdtest change or for some other reason.

If it's okay, I would just merge this and then rebase #14652 to test it properly?

[AlexWaygood]

Sounds good!

[sharkdp]

Seems to work as intended.