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