tests/netgraph: Tests for ng_vlan_rotate

Test functionality of ng_vlan_rotate(4):

• Rotate 1 to 9 stagged vlans in any possible direction and length
• Rotate random combinations of ethertypes (8100, 88a8, 9100)
• Automatic reverse rotating for backward data flow
• Test too many and too few vlans

Reviewed by: kp (earlier version)
Differential Revision: https://reviews.freebsd.org/D30670

(cherry picked from commit 6b08e68be111d50931b0d30145f8b7e3402decaf)