net tests: fix if_stf:6to4

This test needs to have the loopback interface enabled, or route lookups
for our own IP addresses will fail.

MFC after: 3 weeks
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D33041

(cherry picked from commit 67573b7a39b19e39536a68563fed0d940604e573)