ping: Require root user for pytests

atf_python may use vnet jails for creating an isolated test environment.
Mark these tests that require root user privileges.

Reviewed by: markj
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/863

(cherry picked from commit 4efaf43c6fa76e0daa1ed5ea535d863f15415bf4)