Tests: add basic ust context tests for $app, vpid, vuid, vgid

Context tracing has very little testing coverage beyond the namespace
tests. This test is initially introduced to help troubleshoot an issue
with application contexts.

However, with the scaffolding now in place, it's trivial to exercise
some other context types.

This also adds a basic framework to write tests in Python.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I85be842fab252d8b853392d3f742b1461a69f0fe