Add a wait_for_catchup() before immediate stop of a test master.

Per buildfarm member hoverfly, a slow walsender could make the test
fail.  Back-patch to v10, where the test was introduced.

Discussion: https://postgr.es/m/20200414013849.GA886648@rfd.leadboat.com

src/test/recovery/t/010_logical_decoding_timelines.pl

@@ -131,6 +131,7 @@ cmp_ok(
 	'xmin on physical slot must not be lower than catalog_xmin');
 
 $node_master->safe_psql('postgres', 'CHECKPOINT');
+$node_master->wait_for_catchup($node_replica, 'write');
 
 # Boom, crash
 $node_master->stop('immediate');