pfctl tests: Test fairq configuration

We used to have a bug where pfctl could crash setting fairq queues. Test
this case and ensure it does not crash pfctl.

Reviewed by: donner
MFC after: 1 week
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D30348

(cherry picked from commit 9938fcaa6565a660c555a0e9c712842ba1a2d31c)