Improve recently-added test reliability

Improve recently-added test reliability

Commit 59be1c942a47 already tried to make
src/test/recovery/t/033_replay_tsp_drops more reliable, but it wasn't
enough.  Try to improve on that by making this use of a replication slot
to be more like others.  Also, don't drop the slot.

Make a few other stylistic changes while at it.  It's still quite slow,
which is another thing that we need to fix in this script.

Backpatch to all supported branches.

Discussion: https://postgr.es/m/349302.1659191875@sss.pgh.pa.us