Revert "TTY: pty, remove superfluous ptm test"

This reverts commit a50f724a.

Sasha reported that this causes problems, so revert it.

Cc: Alan Cox <alan@linux.intel.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Sasha Levin <levinsasha928@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>