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