Buildfarm members kittiwake, tadarida and snapper began to fail
frequently when commits 3cd9c3b921977272e6650a5efbeade4203c4bca2 and
f47ed79cc8a0cfa154dc7f01faaf59822552363f added tests of concurrency, but
the problem was reachable before those commits.  Back-patch to v10 (all
supported versions).

Discussion: https://postgr.es/m/20220116210241.GC756210@rfd.leadboat.com