swift234

More resilient class layout tests

Looks like subclassing classes with resiliently-sized properties works,
as long as the subclass is @_fixed_layout, so let's ensure that's tested.

We don't want @_fixed_layout classes to be a thing though, and we still
can't handle changes to the number of stored properties in a base class,
so a couple of tests are disabled until I land some more patches.