ping tests: Run tests unprivileged inside a vnet

[jlduran]

Summary

In order to create vnet jails, atf_python needs root privileges.
However, once the vnet is created, its privileges inside the vnet can be
demoted, as these tests should not require root in order to pass.

Fixes:  4efaf43c6fa76e0daa1ed5ea535d863f15415bf4