Tests: flt.utils.muxer: add message ordering tests

* Add a test that checks the basic ordering of messages based on
  timestamps.

* Add a test that checks the basic ordering of messages with the same
  timestamps but different trace name, stream class id, or stream id.

* Add tests that check the ordering of messages with the same timestamps
  and types.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Change-Id: I217119a00828288d0a2471c26b387a670e62608a
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1726
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>