[raorjun]

What

make test ran the 1100 tests one at a time. It took about 6 minutes locally and 5 to 9 minutes in CI. This PR adds pytest-xdist and runs the suite with -n auto.

test_bobsim_sources_do_not_reference_legacy_boblib_generation_layout calls git ls-files. Git does not work in a container that mounts a git worktree, so the test failed there. It now skips with the git error.

How to check

• make test in the BobSim image: 1061 passed and 40 skipped in 2:14. On main it was 1061 passed and 1 failed in 5:59.
• The serial and parallel runs give the same results.
• Compare the Tests job time on this PR with recent main runs.

Notes

A local venv needs pip install -r requirements.txt once to get pytest-xdist.