Tests: src.ctf.lttng-live: add support for beacon injections

Tests: src.ctf.lttng-live: add support for beacon injections

Add a 'beacons' field to the sessions configuration accepted
by lttng_live_server.py. 'beacons' is expected to be a list
of clock snapshots (in cycles) at which live inactivity beacons
should be sent to a live client.

From a protocol standpoint, an inactivity beacon is a "special" type
of index entry. They are inserted in the index list (_entries)
so that they can simply be sent in response to a
LttngLiveViewerGetNextDataStreamIndexEntryCommand in the same
way typical index entries are processed.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I1f86cf369a5115af6787e9a9d52469d16f1ab3aa
Reviewed-on: https://review.lttng.org/c/babeltrace/+/3618
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>