Tests: Add "linking" ust regression test

Based on the "demo" test formerly part of lttng-ust. Tests userspace
tracing with directly, statically and dynamically linked providers. Also
meant as an exemple of building and tracing an application with all
types of provider linking.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>