Allow Context subclasses in acceptance tests to access parent actor

Although uncommon, in some cases a Context may need to be extended (for
example, to override a step defined in the server with a specific
behaviour in the acceptance tests of an app); in those cases the
subclass should be able to access the actor attribute defined in the
Context it is extending.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>