if_ovpn tests: allow these tests to run in parallel

Now that kyua can isolate tests in their own vnets we can run these tests in
parallel.

Reviewed by: markj
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D46600

(cherry picked from commit a05decec2d194821f8bf5c8f2ae72063de0c87a8)

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