Add conflict-free invariant check to Configuration

Add conflict-free invariant check to Configuration

When adding events to a configuration, we want to
ensure that the configuration remains both a)
causally closed and b) conflict-free. The latter
required that the tests be modified as they effectively
assumed that the transitions associated with each
event were independent of one another. Since the
`Transition` base class is dependent by default, this
was at odds with the assumptions