Refactor routine to find single log content pattern in TAP tests

The same routine to check if a specific pattern can be found in the
server logs was copied over four different test scripts.  This refactors
the whole to use a single routine located in PostgreSQL::Test::Cluster,
named log_contains, to grab the contents of the server logs and check
for a specific pattern.

On HEAD, the code previously used assumed that slurp_file() could not
handle an undefined offset, setting it to zero, but slurp_file() does
do an extra fseek() before retrieving the log contents only if an offset
is defined.  In two places, the test was retrieving the full log
contents with slurp_file() after calling substr() to apply an offset,
ignoring that slurp_file() would be able to handle that.

Backpatch all the way down to ease the introduction of new tests that
could rely on the new routine.

Author: Vignesh C
Reviewed-by: Andrew Dunstan, Dagfinn Ilmari Mannsåker, Michael Paquier
Discussion: https://postgr.es/m/CALDaNm0YSiLpjCmajwLfidQrFOrLNKPQir7s__PeVvh9U3uoTQ@mail.gmail.com
Backpatch-through: 11

src/test/perl/PostgresNode.pm

@@ -1962,6 +1962,21 @@ sub issues_sql_like
 
 =pod
 
+=item $node->log_contains(pattern, offset)
+
+Find pattern in logfile of node after offset byte.
+
+=cut
+
+sub log_contains
+{
+	my ($self, $pattern, $offset) = @_;
+
+	return TestLib::slurp_file($self->logfile, $offset) =~ m/$pattern/;
+}
+
+=pod
+
 =item $node->run_log(...)
 
 Runs a shell command like TestLib::run_log, but with connection parameters set

src/test/recovery/t/033_replay_tsp_drops.pl

@@ -132,21 +132,11 @@ while ($max_attempts-- >= 0)
 {
 	last
 	  if (
-		find_in_log(
-			$node_standby, qr!WARNING: ( [A-Z0-9]+:)? creating missing directory: pg_tblspc/!,
+		$node_standby->log_contains(
+			qr!WARNING: ( [A-Z0-9]+:)? creating missing directory: pg_tblspc/!,
 			$logstart));
 	usleep(100_000);
 }
 ok($max_attempts > 0, "invalid directory creation is detected");
 
 done_testing();
-
-# find $pat in logfile of $node after $off-th byte
-sub find_in_log
-{
-	my ($node, $pat, $off) = @_;
-
-	my $log = PostgreSQL::Test::Utils::slurp_file($node->logfile, $off);
-
-	return $log =~ m/$pat/;
-}