Minor fixes to test_data_generation.py

[gilles-peskine-arm]

This is a collection of minor fixes to issues I found in Python scripts near the end of my review of https://github.com/Mbed-TLS/mbedtls/pull/6093. I didn't want to hold that PR for minor, mostly preexisting issues, hence this follow-up.

• Less ambiguous name for test_generation.py.
• Fix paths when not invoked from the mbedtls root.
• Docstring that should have been a comment.
• Use relative imports (fixes e.g. running python3 -m unittest scripts/mbedtls_dev/psa_storage.py).
• Unify the 4 copies of check_repo_path (reported by pylint when not running with -j).
• Don't run pylint with -j since that makes it not signal code that's duplicated in different modules.

Changelog: no (no user-visible bugs were fixed).

Backport: TODO (once #6093 is backported).