Minor test fixes in 035_standby_logical_decoding.pl

Import usleep, which, due to an oversight in oversight in commit
48796a98d5 was used but not imported.

Correct the comparison string used in two logfile checks. Previously, it
was incorrect and thus the test could never have failed.

Also wordsmith a comment to make it clear when hot_standby_feedback is
meant to be on during the test scenarios.

Reported-by: Melanie Plageman <melanieplageman@gmail.com>
Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Masahiko Sawada <sawada.mshk@gmail.com>
Discussion: https://postgr.es/m/flat/CAAKRu_YO2mEm%3DZWZKPjTMU%3DgW5Y83_KMi_1cr51JwavH0ctd7w%40mail.gmail.com
Backpatch-through: 16

src/test/recovery/t/035_standby_logical_decoding.pl

@@ -8,6 +8,7 @@ use warnings FATAL => 'all';
 
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
+use Time::HiRes qw(usleep);
 use Test::More;
 
 if ($ENV{enable_injection_points} ne 'yes')
@@ -623,7 +624,7 @@ ok( $stderr =~
 	  /ERROR:  cannot copy invalidated replication slot "vacuum_full_inactiveslot"/,
 	"invalidated slot cannot be copied");
 
-# Turn hot_standby_feedback back on
+# Set hot_standby_feedback to on
 change_hot_standby_feedback_and_wait_for_xmins(1, 1);
 
 ##################################################
@@ -754,12 +755,12 @@ wait_until_vacuum_can_remove(
 
 # message should not be issued
 ok( !$node_standby->log_contains(
-		"invalidating obsolete slot \"no_conflict_inactiveslot\"", $logstart),
+		"invalidating obsolete replication slot \"no_conflict_inactiveslot\"", $logstart),
 	'inactiveslot slot invalidation is not logged with vacuum on conflict_test'
 );
 
 ok( !$node_standby->log_contains(
-		"invalidating obsolete slot \"no_conflict_activeslot\"", $logstart),
+		"invalidating obsolete replication slot \"no_conflict_activeslot\"", $logstart),
 	'activeslot slot invalidation is not logged with vacuum on conflict_test'
 );