Fix: add missing synchronization point for before app test case

Fix: add missing synchronization point for before app test case

Fixes a race where the application could generate all its events before
trace start.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Conflicts:
	tests/utils/testapp/gen-ust-events/gen-ust-events.c

Conflicts:
	tests/regression/ust/java-jul/test_java_jul
	tests/regression/ust/java-log4j/test_java_log4j

Conflicts:
	tests/regression/tools/streaming/test_ust