Run sphinx-lint against friend projects.

[ezio-melotti]

I noticed that the workflow downloads "friend projects" but doesn't seem to run sphinx-lint against them -- only pytest. However, it appears that none of those repos/dirs have any docs-related tests (or any tests at all), so nothing is done with that.

I'm assuming the intention was to run sphinx-lint against them, so I'm opening this draft PR to see what happens.

[ezio-melotti]

I just noticed that [download-more-tests.sh](https://github.com/sphinx-contrib/sphinx-lint/blob/main/download-more-tests.sh) was just added recently, so it might still be a work-in-progress 🙃.

The last commit seem to pass though, but I'm going to double check if there is any issue. (cc @JulienPalard)

[ezio-melotti]

I tried locally and verified that the projects are cloned correctly and none of them have any error. Passing --enable=default-role results in several errors, proving that the command works correctly.

[ezio-melotti]

#46 revealed several failures that only happen when --enable=default-role is passed, that are not currently exposed by this PR.
The tests should probably use the same commands used in each of those repos.

[JulienPalard]

pytest run tests against friend projects here:

sphinx-lint/tests/test_sphinxlint.py

Line 76 in 30225a3

def test_sphinxlint_friend_projects_shall_pass(file, capsys):

I did it like this because the first version was downloading friends projects under fixtures/xpass/ so no lines of code was needed to test them. But it was a pain to see them in git status :D

[JulienPalard]

I added documentation in download-more-tests.sh and a docstring in test_friends.py.