t/lib-terminal: allow TTY tests to run under recent Mac OS

TTY tests were previously skipped on all Mac OS systems to avoid a
bug where reading from pty master occasionally hung.

The bug has since been found not to be reproducible under Mac OS
10.9 and 10.10.1, so allow TTY tests under Mac OS 10.9 (Mavericks)
and more recent versions.

Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: John Szakmeister <john@szakmeister.net>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Mike Blume <blume.mike@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>