src.ctf.fs: deterministically sort trace's DS file groups

src.ctf.fs: deterministically sort trace's DS file groups

This patch makes a `src.ctf.fs` component deterministically sort its
trace's DS file groups. The order of the DS file group array directly
influences:

* The order of the output ports.
* Which DS file group gets which stream ID if the trace's packet headers
  do not contain a `stream_instance_id` field (automatic stream IDs).

It's important that, whatever the build or file system, the component
always assigns the same IDs to the same DS file group for a given trace.
This can help debugging and testing.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I105f44e7885a2974ace4d62abe0ee5384316b350
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2079
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>