netpfil tests: run in parallel

Run these tests in their own (vnet) jail so we don't have to worry about IP
range or jail name conflicts.

Reviewed by: markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46040

(cherry picked from commit 4f752a1583dc97d166caae7f844bf42715e99978)

Approved by: kp (mentor), markj (mentor)