Add bindings/python/babeltrace/test_ctf_writer.py test

This test ensures that we can use the legacy Python bindings to create a
CTF trace without caring about timestamp fields (they should be
automatically updated from the stream's class's clock value).

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>