Ignore more environment variables in pg_regress.c

This is similar to the work done in 8279f68 for TestLib.pm, where
environment variables set may cause unwanted failures if using a
temporary installation with pg_regress.  The list of variables reset is
adjusted in each stable branch depending on what is supported.

Comments are added to remember that the lists in TestLib.pm and
pg_regress.c had better be kept in sync.

Reviewed-by: Álvaro Herrera
Discussion: https://postgr.es/m/YMNR9GYDn+fHlMta@paquier.xyz
Backpatch-through: 9.6

src/test/perl/TestLib.pm

@@ -56,6 +56,7 @@ BEGIN
 	delete $ENV{LC_ALL};
 	$ENV{LC_MESSAGES} = 'C';
 
+	# This list should be kept in sync with pg_regress.c.
 	my @envkeys = qw (
 	  PGCLIENTENCODING
 	  PGCONNECT_TIMEOUT