Include `--parallel-threads` jobs in free threading TSAN CI

[colesbury]

Feature or enhancement

The regression test runner can now parallelize test cases to help catch thread-safety related bugs:

• Optionally run Python regression tests in parallel #127933

I'd like to add test suites run with --parallel-threads=N to the existing free threading TSAN CI job. We could also run them in the default build's TSAN CI job, but I don't think that would be useful.

Linked PRs

• gh-129694: Add --parallel-threads TSAN job to CI #129696
• gh-129694: Add test_exceptions to parallel TSAN jobs #129746