bt2: Adapt test_connection.py and make it pass

bt2: Adapt test_connection.py and make it pass

The test_connection test is adapted to account for:

 - Connections can't be in an ended state anymore.
 - We don't need to define equality between connections.
 - Private connections don't exist anymore.

The bindings are also simplified by the fact that private connections
don't exist anymore, and connections can't end.  Otherwise, changes are
straightforward to adapt to the existing API.

Change-Id: I52cb89512df1e8cb0cd88670c6d63e68287556c3
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1280
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>
Tested-by: jenkins