tests: skip terminal output tests on OS X

tests: skip terminal output tests on OS X

On Mac OS X 10.5.0, test_terminal gets stuck reading from the pty
master every once in a while.  To reproduce the problem:

 perl -MIO::Pty -MFile::Copy -e '
	for (my $i = 0;; $i++) {
		my $master = new IO::Pty;
		my $slave = $master->slave;
		if (fork == 0) {
			close $master or die "close: $!";
			open STDOUT, ">&", $slave or die "dup2: $!";
			close $slave or die "close: $!";
			exec("echo", "hi", $i) or die "exec: $!";
		}
		close $slave or die "close: $!";
		copy($master, \*STDOUT) or die "copy: $!";
		close $master or die "close: $!";
		wait;
	}
 '

It blocks after 7000 iterations or so in sysread().  The relevant
sysread() call is the second call by the parent, which presumably
executes before the child dies but after the parent has read all
output from there.

Since this is an intermitent problem, the quick check of terminal
support in lib-terminal doesn't catch it.  Skip these tests on the Mac
for now.

Noticed-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>