gh-129694: Add --parallel-threads TSAN job to CI (gh-129696)

For now, this just adds a single test suite to the TSAN CI to be run
with `--parallel-threads`.

.github/workflows/reusable-tsan.yml

@@ -74,6 +74,9 @@ jobs:
       run: make pythoninfo
     - name: Tests
       run: ./python -m test --tsan -j4
+    - name: Parallel tests
+      if: fromJSON(inputs.free-threading)
+      run: ./python -m test --tsan-parallel --parallel-threads=4 -j4
     - name: Display TSAN logs
       if: always()
       run: find "${GITHUB_WORKSPACE}" -name 'tsan_log.*' | xargs head -n 1000