tests: Cope with WARNINGs during failed CREATE DB on windows

tests: Cope with WARNINGs during failed CREATE DB on windows

The test added in 93bc3d75d8e sometimes fails on windows, due to warnings like
WARNING:  some useless files may be left behind in old database directory "base/16514"

The reason for that is createdb_failure_callback() does not ensure that there
are no open file descriptors for files in the partially created,
to-be-dropped, database. We do take care in dropdb(), but that involves
waiting for checkpoints and a ProcSignalBarrier, which we probably don't want
to do in an error callback.  This should probably be fixed one day, but for
now 001_aio.pl needs to cope.

Per buildfarm animals fairywren and drongo.

Discussion: https://postgr.es/m/uc62i6vi5gd4bi6wtjj5poadqxolgy55e7ihkmf3mthjegb6zl@zqo7xez7sc2r