ping tests: Run tests unprivileged inside a vnet

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.

Reviewed by: markj
Approved by: emaste (mentor)
Fixes: 4efaf43c6fa7 ("ping: Require root user for pytests")
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42175