It can be useful to check whether a ClutterActorIter is currently valid,
i.e. if the iterator has been initialized *and* if the actor to which it
refers to hasn't been updated.
We can also use the is_valid() method in the conformance test suite to
check that initialization has been successful, and that changing the
children list through the ClutterActorIter API leaves the iterator in a
valid state.
It's the conformance test suite: there's no need to namespace the files,
just like there's no need to namespace the units.
This commit does not change the Cogl tests: they will be moved to Cogl
over time, and it's easier to do if we leave them as they are.
