Fix: tests: add missing wait, document missing synchro

Fix: tests: add missing wait, document missing synchro

Move all wait ${!} that target a single process to "wait", to minimize
the chances to forget some background process in the future.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Conflicts:
	tests/regression/ust/java-log4j/test_java_log4j

Conflicts:
	tests/regression/tools/streaming/test_ust
	tests/regression/ust/before-after/test_before_after
	tests/regression/ust/buffers-pid/test_buffers_pid
	tests/regression/ust/multi-session/test_multi_session