selftests: tty: add selftest for tty timestamp updates

Add new test case which checks that timestamp updates on actual terminal
character device (e.g. /dev/pts/0) happen even if the terminal is
accessed via magic /dev/tty file.

Signed-off-by: Michal Sekletar <msekleta@redhat.com>
Message-ID: <20230613172107.78138-2-msekleta@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>