"One way to fix this would be to add a WithSignalHandler(signal, handler) context manager to test.support that would replace the signal handler upon entry, and re-register faulthandler's handler upon exit."

faulthandler.dump_traceback_later() is registered before each test, in test.regrtest.runtest().

You may reinstall signal handlers before each test, or fix signal.getsignal() and signal.signal(). I don't have a preference.