tests/netgraph: Inital framework for testing libnetgraph
Provide a framework of functions to test various netgraph modules.
Tests contain:
- creating, renaming, and destroying nodes
- connecting and removing hooks
- sending and receiving data
- sending ASCII messages
- errors can be passed for indiviual inspection or fail the test
Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30629