Don't run timers in input-pending-p. Its new check-timers param

Don't run timers in input-pending-p.  Its new check-timers param
provides the prior behavior..
* src/keyboard.c (Finput_pending_p): Accept optional check-timers
param.
* lisp/subr.el (sit-for): Call (input-pending-p t) so as to behave
as before.
* test/automated/timer-tests.el: New file.  Tests that (sit-for 0)
allows another timer to run.

Fixes: debbugs:15045