Race conditions with TAP test for syncrep

Hi all,

Alvaro has reported a rather rare buildfarm failure involving
007_sync_rep.pl to which I have responded here:
https://www.postgresql.org/message-id/20190613060123.GC1643@paquier.xyz

The buildfarm failure is here:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=chipmunk&dt=2019-05-12%2020%3A37%3A11

It involves a race condition related to the way the standbys of the
test are stopped and restarted to ensure that they appear in the
correct order in the WAL sender array of the primary, but feel free to
look at the message above for all the details.

Attached is a patch to improve the stability of the test.  The fix I
am proposing is very simple: in order to make sure that a standby is
added into the WAL sender array of the primary, let's check after
pg_stat_replication after a standby is started.  This can be done
consistently with a small wrapper in the tests.

Any thoughts?
--
Michael