Why is src/test/modules/committs/t/002_standby.pl flaky?

Hi,

There's a wait for replay that is open coded (instead of using the
wait_for_catchup() routine), and sometimes the second of two such
waits at line 51 (in master) times out after 3 minutes with "standby
never caught up". It's happening on three particular Windows boxes,
but once also happened on the AIX box "tern".

branch | animal | count
---------------+-----------+-------
HEAD | drongo | 1
HEAD | fairywren | 8
REL_10_STABLE | drongo | 3
REL_10_STABLE | fairywren | 10
REL_10_STABLE | jacana | 3
REL_11_STABLE | drongo | 1
REL_11_STABLE | fairywren | 4
REL_11_STABLE | jacana | 3
REL_12_STABLE | drongo | 2
REL_12_STABLE | fairywren | 5
REL_12_STABLE | jacana | 1
REL_12_STABLE | tern | 1
REL_13_STABLE | fairywren | 3
REL_14_STABLE | drongo | 2
REL_14_STABLE | fairywren | 6

https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-30%2014:42:30
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-30%2013:13:22
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-30%2006:03:07
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-22%2011:37:37
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-22%2010:46:07
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-22%2009:03:06
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-17%2004:59:17
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-17%2003:59:51
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-16%2004:37:58
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-15%2009:57:14
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-15%2002:38:43
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-14%2020:42:15
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-14%2012:08:41
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-14%2000:35:32
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-13%2023:40:11
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-13%2022:47:25
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-09%2006:59:10
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-09%2006:04:04
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-09%2001:36:09
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-08%2019:20:35
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-08%2018:04:28
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-08%2014:12:32
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-12-08%2011:15:58
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2021-12-08%2004:04:22
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2021-12-03%2017:31:49
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-11-11%2015:58:55
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-10-02%2022:00:17
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-09-09%2005:16:43
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-08-24%2004:45:09
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tern&dt=2021-07-17%2010:57:49
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2021-06-12%2016:05:32
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2021-02-07%2012:59:43
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2020-03-24%2012:49:50
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2020-02-01%2018:00:27
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2020-02-01%2017:26:27
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2020-01-30%2023:49:49
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=jacana&dt=2019-12-22%2014:19:02
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-12-13%2000:12:11
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-12-09%2006:02:05
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-12-06%2003:07:42
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-11-02%2014:41:04
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-25%2013:12:08
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-24%2013:12:41
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-23%2023:10:00
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-23%2018:00:39
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-22%2015:05:57
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-18%2013:29:49
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-16%2014:54:46
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-15%2014:21:11
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-14%2013:15:07
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-13%2014:19:41
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=drongo&dt=2019-10-12%2016:32:06
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=fairywren&dt=2019-10-10%2013:12:09