Tests: unit testing for backward compatibility of group-output-by-session

These unit tests aim to reproduce the stream path received by
lttng-relayd when dealing with a producer using a protocol version < 2.11.

We then pass them through the transform function for
--group-output-by-session used when producer are using protocol version
greater or equal to 2.4 and small than 2.11.

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