This part of the test was included originally in 4e87c483, but got
reverted as of f9c1b8db because of timing issues in the test, where,
after more analysis, we found that the standbys may not have recovered
from the archives all the segments needed by the test.  This stabilizes
the test by making sure that standbys replay up to the position returned
by pg_switch_wal(), meaning that all segments are recovered before the
manual checkpoint that tests WAL segment recycling and its interactions
with archive status files.

Author: Jehan-Guillaume de Rorthais
Reviewed-by: Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/20200424034248.GL33034@paquier.xyz