Close #12363: fix a race condition in siginterrupt() tests

Close #12363: fix a race condition in siginterrupt() tests

The previous tests used time.sleep() to synchronize two processes. If the host
was too slow, the test could fail.

The new tests only use one process, but they use a subprocess to:

 - have only one thread
 - have a timeout on the blocking read (select cannot be used in the test,
   select always fail with EINTR, the kernel doesn't restart it)
 - not touch signal handling of the parent process