In the pg_upgrade test suite, remove and recreate "tmp_check".

This allows "vcregress upgradecheck" to pass twice in immediate
succession, and it's more like how $(prove_check) works.  Back-patch to
9.5, where pg_upgrade moved to src/bin.

Discussion: https://postgr.es/m/20190520012436.GA1480421@rfd.leadboat.com