This allows us to skip it in Cygwin, where it's reportedly flaky because
of platform bugs or something.
Backpatch to 17, where the test was introduced by commit 2466d6654f85.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Discussion: https://postgr.es/m/e4d0cb33-6be5-e4d5-ae49-9eac3ff2b005@gmail.com
