Tests: environment: base WaitTraceTestApplication on gen-ust-events

Tests: environment: base WaitTraceTestApplication on gen-ust-events

WaitTraceTestApplication wraps gen-ust-nevents, but that test app offers
few synchronization points compared to gen-ust-events.

The synchronization points to determine that the application has reached
its `main()` and just before emiting its first event are added to
gen-ust-events. WaitTraceTestApplication is adapted to use those new
options.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Idf717fbaa9108d48a3f7d2b26946a4e5c5dfffd5