tests/netgraph: Tests for ng_vlan_rotate

[donner]

Summary

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)
• Provide too few or too many vlans

Test Plan

# kyua test vlan_rotate
vlan_rotate:basic  ->  passed  [4.400s]
vlan_rotate:ethertype  ->  passed  [1.087s]
vlan_rotate:minmax  ->  passed  [1.936s]
vlan_rotate:reverse  ->  passed  [4.401s]
vlan_rotate:typeether  ->  passed  [1.086s]

[donner]

• Add reverse tests, going the other direction reverses the rotation

[donner]

• Factor out common routines.

[donner]

• Style fixes

[donner]

Rebase

[donner]

• Test too few or too many vlans