Try to fix pg_upgrade test on Windows, again.

Try to fix pg_upgrade test on Windows, again.

Further to commit 54e72b66e, if rmtree() fails while cleaning up in
pg_upgrade, try again.  This gives our Windows unlink() wrapper a chance
to reach its wait-for-the-other-process-to-go-away logic, if the first
go around initiated the unlink of a file that a concurrently exiting
program still has open.

Discussion: https://postgr.es/m/CA%2BhUKGKCVy2%3Do%3Dd8c2Va6a_3Rpf_KkhUitkWCZ3hzuO2VwLMXA%40mail.gmail.com