Honor TEMP_CONFIG in TAP suites.

The buildfarm client uses TEMP_CONFIG to implement its extra_config
setting.  Except for stats_temp_directory, extra_config now applies to
TAP suites; extra_config values seen in the past month are compatible
with this.  Back-patch to 9.6, where PostgresNode was introduced, so the
buildfarm can rely on it sooner.

Reviewed by Andrew Dunstan and Tom Lane.

Discussion: https://postgr.es/m/20181229021950.GA3302966@rfd.leadboat.com

src/bin/pg_ctl/t/001_start_stop.pl

@@ -24,6 +24,8 @@ command_ok([ $ENV{PG_REGRESS}, '--config-auth', "$tempdir/data" ],
 	'configure authentication');
 open my $conf, '>>', "$tempdir/data/postgresql.conf";
 print $conf "fsync = off\n";
+print $conf TestLib::slurp_file($ENV{TEMP_CONFIG})
+  if defined $ENV{TEMP_CONFIG};
 if (!$windows_os)
 {
 	print $conf "listen_addresses = ''\n";

src/test/perl/PostgresNode.pm

@@ -452,7 +452,17 @@ sub init
 	print $conf "log_statement = all\n";
 	print $conf "log_replication_commands = on\n";
 	print $conf "wal_retrieve_retry_interval = '500ms'\n";
-	print $conf "port = $port\n";
+
+	# If a setting tends to affect whether tests pass or fail, print it after
+	# TEMP_CONFIG.  Otherwise, print it before TEMP_CONFIG, thereby permitting
+	# overrides.  Settings that merely improve performance or ease debugging
+	# belong before TEMP_CONFIG.
+	print $conf TestLib::slurp_file($ENV{TEMP_CONFIG})
+	  if defined $ENV{TEMP_CONFIG};
+
+	# XXX Neutralize any stats_temp_directory in TEMP_CONFIG.  Nodes running
+	# concurrently must not share a stats_temp_directory.
+	print $conf "stats_temp_directory = 'pg_stat_tmp'\n";
 
 	if ($params{allows_streaming})
 	{
@@ -478,6 +488,7 @@ sub init
 		print $conf "max_wal_senders = 0\n";
 	}
 
+	print $conf "port = $port\n";
 	if ($use_tcp)
 	{
 		print $conf "unix_socket_directories = ''\n";