Test: prevent the spawning of a daemonized sessiond

Sets the default lttng-sessiond path to /bin/true to prevent the spawning
of a daemonized sessiond. This is necessary since 'lttng create' will spawn
its own sessiond if none is running. It also ensures that 'lttng create'
fails when no sessiond is running.

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