Fix atimer setting and overdue expiration (bug#55628)

Fix atimer setting and overdue expiration (bug#55628)

* src/atimer.c (set_alarm): If the atimer has already expired, signal
it right away instead of postponing it further.  Previously this could
occur repeatedly, blocking atimers indefinitely.
Also only use `alarm` as fallback if `setitimer` is unavailable, not
both at the same time (which makes no sense, and they both typically
use the same mechanism behind the curtains).

* test/src/eval-tests.el (eval-tests/funcall-with-delayed-message):
New test, verifying proper functioning of funcall-with-delayed-message
which also serves as test for this bug (which also caused
debug-timer-check to fail, but that test is only run when Emacs is
built with enable-checking).