wg: Re-add basic if_wg(4) tests.

These were originally added in commit
74ae3f3e33b810248da19004c58b3581cd367843 but have been updated to use
wg(8).

Reviewed by: kevans, markj
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37400

(cherry picked from commit 8fb9739615da8922fbd8988ccd77093d4ca335ba)