CI: Treat mdtest Markdown files as code (#14653)

## 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

Author: sharkdp

.github/workflows/ci.yaml

@@ -82,6 +82,7 @@ jobs:
             code:
               - "**/*"
               - "!**/*.md"
+              - "crates/red_knot_python_semantic/resources/mdtest/**/*.md"
               - "!docs/**"
               - "!assets/**"