Maybe BF "timedout" failures are the client script's fault?

We've been assuming that all the "timedout" failures on BF member
fruitcrow were due to some wonkiness in the GNU/Hurd platform.
I got suspicious about that though after noticing that there are
a small number of such failures on other animals, eg [1][2][3].
In each case, the failure message claims it waited a good long
time, which is at variance with the actually observed runtime.
For instance [1] says "timed out after 14400 secs", but the
actual total test runtime is only 01:24:28 according to the
summary at the top of the page.

Looking into the buildfarm client, I realized that it's assuming that
"sleep($wait_time)" is sufficient to wait for $wait_time seconds.
However, the Perl docs point out that sleep() can be interrupted by a
signal. So now I'm suspicious that many of these failures are caused
by a stray signal waking up the wait_timeout thread prematurely.
GNU/Hurd might just be more prone to that than other platforms.

I propose the attached patch to the BF client to try to make this
more robust.

regards, tom lane

[1] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=ovenbird&dt=2025-11-14%2009%3A21%3A05
[2] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=conchuela&dt=2025-10-17%2018%3A32%3A07
[3] https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=opaleye&dt=2026-01-08%2023%3A07%3A37