Tests: introduce WaitTraceTestApplicationGroup

Tests: introduce WaitTraceTestApplicationGroup

A new test helper, WaitTraceTestApplicationGroup, is added to make it
easier to write test that require a group of applications.

A WaitTraceTestApplicationGroup can be used to:
  - launch a given number of applications,
  - make them trace (almost) all at once,
  - wait for the group of applications to complete

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I18023f1c507993a3e5647f87cd74ead3be392163