tests: gen-ust-events: add touch and wait sync points before exit.

tests: gen-ust-events: add touch and wait sync points before exit.

Allows an app to linger until the wait file is created and signals that
the app is just before the exit.

This is mostly useful for per-pid tracing where trace buffers are
cleaned on application teardown.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Change-Id: I37db4c118ebc759ce97c5de8901cad7b39ac1e01
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>