Parallel test execution examples might deadlock

[ngoldbaum]

The examples in this section might lead to deadlocks if for whatever reason creating a thread raises an error or any other action raises an error before all threads are able to call barrier.wait():

https://py-free-threading.github.io/debugging/#writing-explicitly-concurrent-test-cases

We should update these examples to spawn threads inside a generic try/except and explicitly abort the barrier in the error case.

See numpy/numpy#28290 (comment) where I ran into such a deadlock inside the NumPy CI.