Failure in subscription test 004_sync.pl

Hi all,

wrasse has just failed with what looks like a timing error with a
replication slot drop:
https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=wrasse&dt=2021-06-12%2006%3A16%3A30

Here is the error:
error running SQL: 'psql:<stdin>:1: ERROR:  could not drop replication
slot "tap_sub" on publisher: ERROR:  replication slot "tap_sub" is
active for PID 1641'

It seems to me that this just lacks a poll_query_until() doing some
slot monitoring?  I have not checked in details if we need to do that
in more places.  The code path that failed has been added in 7c4f524
from 2017.

Thanks,
--
Michael