atf_python: vnet: Use absolute paths within helpers

Usually tests are run in sterile environments; however, there is a
slight chance that the PATH overrides the utilities used for testing.

Pedantically use absolute paths, even inside VNETs, to avoid ambiguity.

Chiefly, jexec -> /usr/sbin/jexec, and ifconfig -> /sbin/ifconfig.

Reviewed by: markj
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/865