test_graph.py: add bt2.Graph.add_component() logging level parameter tests

test_graph.py: add bt2.Graph.add_component() logging level parameter tests

GraphTestCase.test_add_component_logging_level() is pretty much the same
test as GenericComponentTestCase.test_logging_level(), but the former's
purpose is to test the bt2.Graph.add_component() part of this, while the
latter tests the component's public `logging_level` property.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I9c82e8a7ab6a733810eabfee5ccef66cd619a610
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1456
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Francis Deslauriers <francis.deslauriers@efficios.com>