tests: Add example project for offline builds

[nbraud]

• [x] Add an example project for bork to build in tests without network access As a bonus, that's makes for a pretty-fast instance of test_repo

Did some refactorings of tests/ in the process:

• [x] Refactored test_repos with parametrized fixtures
• [x] Added helpers.chdir, refactored some more with that

[nbraud]

Not too sure what to do about the lint failure in tests/; the lints are erroneous:

• W0621: “Redefining name 'project_src' from outer scope” will trigger on any fixture which uses another fixture defined in the same file;
• W0613: “Unused argument 'project'” will trigger on anything using the project fixture and only relying on it setting the working directory.

I guess for now I'll just sprinkle noqa annotations, since:

• pylint doesn't seem to disable those two lints only in the tests/ directory;
• disabling them globally seems undesirable;
• same for not linting tests/ at all.