isisd, yang: move IS-IS interface state data to the appropriate place

isisd, yang: move IS-IS interface state data to the appropriate place

Now that the "frr-interface" list has a "state" container, move the
IS-IS interface state nodes underneath it using a new augmentation.

Also, update the IS-IS SR topotest to account for this change. Make
use of symlinks where possible to avoid having multiple files with
the same content.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>