tests/netgraph: Tests for ng_bridge

Test functionality of ng_bridge(4):

• replicating traffic to anything but the sending hook
• persistence
• detect loops
• unicast to only one link of many
• stretch to implementation limits on broadcast

Reviewed by: kp
Differential Revision: https://reviews.freebsd.org/D30647
Differential Revision: https://reviews.freebsd.org/D30699

(cherry picked from commit 61814702398ce29430b2bef75cbdd6fd2c07ad12)
(cherry picked from commit 5554abd9cc9702af30af90925b33c5efff4e7d88)