bug#55628: debug-timer-check fails on macOS 11.6.6

[Eli Zaretskii]

> Cc: 55628@debbugs.gnu.org
> From: Mattias Engdegård <mattiase@acm.org>
> Date: Wed, 25 May 2022 15:05:32 +0200
> 
> Thanks for checking. Apparently pending_signals is never set; 
> handle_alarm_signal is never called.
> More digging shows that set_alarm always sets the interval timer to at least 
> 1 ms in the future even if the timer has expired.
> 
> The attached patch makes set_alarm signal SIGALRM immediately if the timer is 
> due. We could call the signal handler directly but this way it should work 
> correctly if the signal is masked at the moment. What do you think?

I'd prefer not to change the logic of set_alarm just to have a
debugging aid working.  Can we come up with a more compatible change,
which doesn't affect calls to set_alarm except during this test?

Thanks.