Tests: Fix possible segfault in health check thread exit test

The testpoints in the thread_manage_clients and thread_manage_apps are
called after a rcu_register_thread() call. The testpoints were not
unregistering their respective thread which could later on cause a
segfault when shutting down the session daemon.

Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>