Tests: utils.sh: pass arbitrary arguments to start_lttng_sessiond*

Tests: utils.sh: pass arbitrary arguments to start_lttng_sessiond*

Allows for easier arguments passing to lttng-sessiond. This is useful
both for debugging manually and passing specific flag as needed in
testing. Passing  "--event-notifier-error-number-of-bucket=X" or "-vvv"
for examples.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I36bd6eaf1c4fc84c3a1e8cfe70a37a46e7130c75