Windows buildfarm members vs. new async-notify isolation test

I see from the buildfarm status page that since commits 6b802cfc7
et al went in a week ago, frogmouth and currawong have failed that
new test case every time, with the symptom

================== pgsql.build/src/test/isolation/regression.diffs ===================
*** c:/prog/bf/root/REL_10_STABLE/pgsql.build/src/test/isolation/expected/async-notify.out    Mon Nov 25 00:30:49 2019
--- c:/prog/bf/root/REL_10_STABLE/pgsql.build/src/test/isolation/results/async-notify.out    Mon Dec  2 00:54:26 2019
***************
*** 93,99 ****
  step llisten: LISTEN c1; LISTEN c2;
  step lcommit: COMMIT;
  step l2commit: COMMIT;
- listener2: NOTIFY "c1" with payload "" from notifier
  step l2stop: UNLISTEN *;

  starting permutation: llisten lbegin usage bignotify usage
--- 93,98 ----

(Note that these two critters don't run branches v11 and up, which
is why they're only showing this failure in 10 and 9.6.)

drongo showed the same failure once in v10, and fairywren showed
it once in v12.  Every other buildfarm animal seems happy.

I'm a little baffled as to what this might be --- some sort of
timing problem in our Windows signal emulation, perhaps?  But
if so, why haven't we found it years ago?

I don't have any ability to test this myself, so would appreciate
help or ideas.

            regards, tom lane