Since reducing pg_ctl's reaction time in commit c61559ec, some
slower buildfarm members have shown erratic failures in this test.
The reason turns out to be that the test assumes synchronous
replication (because it does not provide any lag time for a commit
to replicate before shutting down the servers), but it had only
enabled sync rep in one direction.  The observed symptoms correspond
to failure to replicate the last committed transaction in the other
direction, which can be expected to happen if the shutdown command
is issued soon enough and we are providing no synchronous-commit
guarantees.

Fix that, and add a bit more paranoid state checking at the bottom
of the script.

Michael Paquier and myself

Discussion: https://postgr.es/m/908.1498965681@sss.pgh.pa.us