Add tests for schema init with inheritance

Tries to test the schema initialization with every permutation of orderings for the relevant classes. I failed to find a way to reproduce any of the issues reported post-0.86.3, but this does at least fail without the fixes in both 0.86.1 and 0.86.3, both of which previously had no regression tests.

@alazier 
All else being equal, I'd prefer obj-c code to c++ in our objc tests, unless there is a feature from c++ we actually need - blocks would probably suffice here.