bt2: wrap bt_graph_run_once() (Graph.run_once())

bt2: wrap bt_graph_run_once() (Graph.run_once())

As opposed to Graph.run() which simply returns `None` when it's finished
running, Graph.run_once() needs a way to indicate that the whole graph
is finished. In that case, the method raises `bt2.Stop`. Otherwise,
Graph.run_once() returns `None`.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I8c5908b7b1a265486534a3ec01c9eb813c2cde7c
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1819
Tested-by: jenkins <jenkins@lttng.org>