bt2: refactor test_message_iterator

bt2: refactor test_message_iterator

This patch tries to make test_message_iterator a bit easier to
understand and extend, since the following patches add various tests in
it.

- Split the test class in two, with "seek beginning" tests in their own
  class.
- Rename _setup_seek_beginning_test to _setup_seek_test, as we'll use it
  for seek ns from origin tests.
- Move _create_graph and _setup_seek_test to be top-level
  functions, they don't really benefit from being static methods.  Plus,
  we'll want to share them between the different test classes.
- Make _setup_seek_test take parameters for various optional methods to
  add to the message iterator class (for now, _user_seek_beginning and
  _user_can_seek_beginning).
- Make _setup_seek_test use _create_graph.
- Split test `test_can_seek_beginning` in three: one for when we have
  a _user_can_seek_beginning method, one for when we don't but have a
  _user_seek_beginning method, and the other one for when we have none
  of those.

Change-Id: Ib99965d60a406acde7c0b16f5fde30a268c84673
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/2014
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>