tests/netgraph: Factor out univeral data counter functions

Factor out the data counter helpers for other tests to use.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30699

tests/netgraph: Fix style

Indent all files correctly

MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30692

tests/netgraph: Attribute errors to the caller

Errors raised in the common util functions should raise the location
of their caller to be useful and include the errno description.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30671

tests/netgraph: Allow receiving answers to messages

Add msg_handler in order to receive messages from netgraph nodes to be
tested.

Reviewed by: kp
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D30657

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