Tests: Allow the creation of dummy users in the lttngtest environment

Tests: Allow the creation of dummy users in the lttngtest environment

There are tests that need other user accounts created (e.g. to exercise
per-UID buffers with more than one user). Those accounts may be created
by the test environment and cleaned up on deletion.

An option has been added to the _WaitTraceTestApplication to run the
application as another using `su`.

Change-Id: Ie003e628258fdfbea1972f1f8825c4466fc2792b
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>