In tests, await an LSN no later than the recovery target.

Otherwise, the test fails with "Timed out while waiting for standby to
catch up".  This happened rarely, perhaps only when autovacuum wrote WAL
between our choosing the recovery target and choosing the LSN to await.
Commit b26f7fa6 fixed one case of this.
Fix two more.  Back-patch to 9.6, which introduced the affected test.

Discussion: https://postgr.es/m/20180101055227.GA2952815@rfd.leadboat.com

src/test/recovery/t/003_recovery_targets.pl

@@ -5,8 +5,9 @@ use PostgresNode;
 use TestLib;
 use Test::More tests => 9;
 
-# Create and test a standby from given backup, with a certain
-# recovery target.
+# Create and test a standby from given backup, with a certain recovery target.
+# Choose $until_lsn later than the transaction commit that causes the row
+# count to reach $num_rows, yet not later than the recovery target.
 sub test_recovery_standby
 {
 	my $test_name       = shift;
@@ -70,9 +71,9 @@ my ($lsn2, $recovery_txid) = split /\|/, $ret;
 # More data, with recovery target timestamp
 $node_master->safe_psql('postgres',
 	"INSERT INTO tab_int VALUES (generate_series(2001,3000))");
-$ret =
-  $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn(), now();");
-my ($lsn3, $recovery_time) = split /\|/, $ret;
+my $lsn3 =
+  $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn();");
+my $recovery_time = $node_master->safe_psql('postgres', "SELECT now()");
 
 # Even more data, this time with a recovery target name
 $node_master->safe_psql('postgres',
@@ -86,10 +87,8 @@ $node_master->safe_psql('postgres',
 # And now for a recovery target LSN
 $node_master->safe_psql('postgres',
 	"INSERT INTO tab_int VALUES (generate_series(4001,5000))");
-my $recovery_lsn =
+my $lsn5 = my $recovery_lsn =
   $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn()");
-my $lsn5 =
-  $node_master->safe_psql('postgres', "SELECT pg_current_wal_lsn();");
 
 $node_master->safe_psql('postgres',
 	"INSERT INTO tab_int VALUES (generate_series(5001,6000))");