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

@@ -2470,6 +2470,21 @@ sub log_check
 
 =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