Fix: tests: use SIGSTOP for crash extraction test

Ensure that SIGKILL being received only by some consumer processes don't
trigger clean shutdown of other consumer processes (due to hang up of
communication FD by the session daemon).

Fix this by invoking a SIGSTOP on both sessiond and consumerd, and wait
until we see those process state change to "stopped" before proceeding
to send the SIGKILL.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
CC: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>