Fix: tests: don't use pidof to wait for test apps

Use the bash shell "wait" to wait for all background tasks rather than
the racy "pidof". Indeed, it's possible that applications have been
forked, but not executed yet, when pidof is done, which would therefore
miss applications. Using "wait" from the shell solves this.

If we want to be really strict, we should have sessiond, consumerd, and
relayd export a file containing their own PID, and wait for this instead
of using pidof. But this will be for another fix.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>