testing: handling non-root users with VNETs in pytest-based tests.

testing: handling non-root users with VNETs in pytest-based tests.

Currently isolation and resource requirements are handled directly
 by the kyua runner, based on the requirements specified by the test.
It works well for simple tests, but may cause discrepancy with tests
 doing complex pre-setups. For example, all tests that perform
 VNET setups require root access to properly function.

This change adds additional handling of the "require_user" property
 within the python testing framework. Specifically, it requests
 root access if the test class signals its root requirements and
 drops privileges to the desired user after performing the pre-setup.

Differential Revision: https://reviews.freebsd.org/D37923
MFC after:	2 weeks