Silence assorted annoying test output.

Remove unnecessary chatter about "checking if IO::Socket::UNIX works";
our tests should never print anything on stderr unless there's a
problem.

Add .gitignore entry for temporary directory now being left behind
in src/test/postmaster.

src/test/perl/PostgreSQL/Test/Cluster.pm

@@ -353,7 +353,6 @@ sub raw_connect_works
 	# IO::Socket::UNIX implementation.
 	if ($PostgreSQL::Test::Utils::use_unix_sockets)
 	{
-		diag "checking if IO::Socket::UNIX works";
 		eval {
 			my $sock = $self->raw_connect();
 			$sock->close();

src/test/postmaster/.gitignore

@@ -0,0 +1,2 @@
+# Generated by test suite
+/tmp_check/