gh-143620: Fix race condition in test_traceback_when_child_process_terminates_abruptly

[a12k]

Fixes gh-143620

There is a race condition in test_concurrent_futures.test_process_pool. The test asserts that a BrokenProcessPool exception has a __cause__ immediately after future.result() raises.

In concurrent.futures.process, the _terminate_broken method sets the exception on the future before it attaches the _RemoteTraceback to the __cause__ attr. Sometimes the test thread wakes up and inspects the exception while __cause__ is still None, which leads to a test failure. I was able to repro (see issue for more details) by injecting a delay in _terminate_broken between the set_exception() call and the __cause__ assignment. This causes a 100% failure rate with AssertionError: None is not an instance of _RemoteTraceback.

My proposed fix is to update the test to use support.sleeping_retry to wait for the __cause__ to be populated. This seems to be the pattern elsewhere in the test suite.