tests: Use pytest instead of custom test framework

[bloussou]

This PR is :

• adding linters on python /tests files
• setting up pytest to simplify tests
• fixing setup_dummy_db script to be able to rerun tests after a fail
• fixing RollingStock.import_railjson(force=True) : https://github.com/DGEXSolutions/osrd/pull/3444/files#diff-db79c0d0d826da64bb56e867ba7cca1f5adb239fb926d4e3792c43b0dc14b7f2R108 (causing an issue when tests were launched two times without deleting manually added rolling stocks)
• tests files have been renamed to be detected by pytest as test files, example : get_infra.py -> test_infra.py
• ⚠️ test logic has not been changed ! ⚠️ (this move shows that there are a lot of tests without asserts, they are just checking that API is returning a 200)

To be done later:

• create a fixture per infra and scenario to have "atomic" tests (a test is with a specific db config and should clean its changes after it has been ran).
• Adding assertions to improve testing
• writing more tests 🎉

close #3424
close #3425

[bloussou]

pytest output: https://github.com/DGEXSolutions/osrd/actions/runs/4366456757/jobs/7636494781#step:16:14

[flomonster]

Thanks for this PR!

[eckter]

This is very nice, we should have done this a long time ago