Code review comment for lp://staging/~mpontillo/maas/iprange-notifications--bug-1393936

Added IPv6 test cases (this works with huge IPv6 subnets like 2001::16!); this is ready for another review.

See also:

https://code.launchpad.net/~mpontillo/maas/fix-notification-editing--bug-1680979/+merge/322567

(needed for these notifications to be dismissed properly)