Revert recent changes to 002_pg_upgrade.pl.

The test is proving to be unreliable in the buildfarm, and we neither
agree on how best to fix it nor have time to do so before the upcoming
release. So for now, put things back to the way they were before commit
d498e052b4b84ae21b3b68d5b3fda6ead65d1d4d.

Discussion: http://postgr.es/m/3628089.1659640252@sss.pgh.pa.us
