Fix recovery_min_apply_delay test

Fix recovery_min_apply_delay test

Previously this test was relying too much on WAL replay to occur in the
exact configured interval, which was unreliable on slow or overly busy
servers.  Use a custom loop instead of poll_query_until, which is
hopefully more reliable.

Per continued failures on buildfarm member hamster (which is probably
the only one running this test suite)

Author: Michaël Paquier