bug#25172: 26.0.50; Concurrency feature, sit-for doesn't work (crashing and unexpected behaviour)

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Date: Sun, 11 Dec 2016 17:47:31 +0100
> Cc: 25172@debbugs.gnu.org
> 
> sit-for returns early if input is available.

It doesn't return early for me with that recipe, not with the current
master.  Whatever I do after invoking make-thread, "foo" gets inserted
after 5 sec, even if I lean on some keyboard key.