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