tests: Add an IPv4 loopback address of 127.0.0.1/8 to the lo0
interface by default when creating VNETSs using pytest.

Reviewed By: asomers
Differential Revision: https://reviews.freebsd.org/D38021

(cherry picked from commit 4856aeaaed17726a1dfa5dd6c736e5b0851255b6)