replace ensure_stubs_compile with faster rust test

[senekor]

part of #1839

[senekor]

The Rust test stubs_are_warning_free basically does everything ensure_stubs_compile.sh does, just in parallel (waaay faster). The only differences I found when comparing were:

• The bash script also checked tests, so I added the --tests flag to the Rust test
• On PRs, the bash script only runs on the exercises that were changed. Since all exercises are checked in parallel now, I think we can drop that complexity. It's not needed anymore for speed.

[ellnix]

Read the files multiple times, I don't think anything fell through the cracks. LGTM!

The parallel test in indeed way faster.