It turns out that the instability complained of in commit d3c09b9b
has an embarrassingly simple explanation.  The test script waits for
the standby to flush incoming WAL to disk, but it should wait for
the WAL to be replayed, since we are testing for the effects of that
to be visible.

While at it, use wait_for_catchup instead of reinventing that logic,
and adjust $Test::Builder::Level to improve future error reports.

Back-patch to v12 where the necessary infrastructure came in
(cf. aforesaid commit).  Also back-patch 7d1aa6bf1 so that the
test will actually get run.

Discussion: https://postgr.es/m/2854602.1632852664@sss.pgh.pa.us