This change does not yet do the work necessary to make any tests pass with threading enabled, which will come in future changes.
This approach is broadly inspired by a6d205dd4c/testtools/testsuite.py (L113) and by unittest-ft.
We add a custom TestResult class that batches up any test result actions and applies them under a lock. We also add a custom TestSuite class that runs individual test cases in parallel using a thread-pool.
We need a reader-writer lock to implement a `@jtu.thread_hostile_test` decorator, which we do by adding bindings around absl::Mutex to jaxlib.
PiperOrigin-RevId: 713312937
0389d617c8