In some cases the main thread is not ready to immediately call siglongjmp(),
e.g. if it currently holds a mutex that is later required during
shutdown.
Therefore, we delay handling errors in worker threads until the main
thread performs the next check itself (or the test function ends).
The same issue remains with SIGALRM.
A worker raises SIGUSR1 to inform the main thread that the test fails. The main
thread then starts cancelling workers, but the offending thread should be
terminated immediately to prevent it from test continuation.
Martin Willi