test: fix LanguagesByFilename case & improve logging

[bzz]

1. Fixes a first test failure from Update Linguist to v7.26.0 #167 (I added a wrong fixture in Small improvements #168)
2. refactoring to a single implementation of maybeCloneLinguist() shared between all 3 tests suits that rely on it
3. force Python CI profile to use legacy build system

[bzz]

Annoying as it is, Python CI profiles started to fail with

      /tmp/tmpqwqr2e71.build-temp/_c_enry.c:569:10: fatal error: /home/runner/work/go-enry/go-enry/.shared/libenry.h: No such file or directory

despite having the dependencies fixed and without any changes on our side 😞 (and the logs of the old runs expired so it is hard to debug). Locally Python bindings can be built and tests run (sans bumping pytest version for Python 3.10).

I guess we should just ignore it for now, and fix later when time permits 🤷

[bzz]

After

pip install -r python/requirements.txt
pip install -r python/requirements.dev.txt

a pip -v install -e python produces VERY different output for the same pip 23.2.1 locally and on CI 🤔 .

If no one figures it out before that, I'm planning to take another look next week, and if that doesn't help - create a separate issue for fixing CI Python profiles and merge this as-is.

[bzz]

Merging as Python CI is 🟢 😎