tests/netgraph: Tests for ng_hub

Test functionality of ng_hub(4):

• replicting traffic to anything but the sending hook
• persistence
• an unrestricted loop
• implementation limits with many hooks.

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D30633

(cherry picked from commit 7863faa78ae271017c404c635b2a9d07379d4316)