Intermittent test failures

[rm-hull]

• test_add_edge_callback(): E assert 71 in {}
• test_edge_detected():

E   assert False
E    +    where False = <function edge_detected at 0x7f4cfb0b5aa0>(23)
E    +    where <function edge_detected at 0x7f4cfb0b5aa0> = event.edge_detected

• test_callback_raises_error(): E Failed: DID NOT RAISE <type 'exceptions.RuntimeError'>