CI: Adding tests detecting changed paths in utils/paths.rs

[tesuji]

It caused hard-to-debug test fallings like this: https://github.com/rust-lang-ci/rust/runs/1116251535#step:23:9212
Would be better to catch this earlier before compiling or running tests.

I'm working on an internal lint to do this. Should have a pull request soon.