Rewrite eintr_tester.py to avoid os.fork()

eintr_tester.py calls signal.setitimer() to send signals to the current process
every 100 ms. The test sometimes hangs on FreeBSD. Maybe there is a race
condition in the child process after fork(). It's unsafe to run arbitrary code
after fork().

This change replace os.fork() with a regular call to subprocess.Popen(). This
change avoids the risk of having a child process which continue to execute
eintr_tester.py instead of exiting. It also ensures that the child process
doesn't inherit unexpected file descriptors by mistake.

Since I'm unable to reproduce the issue on FreeBSD, I will have to watch
FreeBSD buildbots to check if the issue is fixed or not.

Remove previous attempt to debug: remove call to
faulthandler.dump_traceback_later().