test subclasses of core PMCS

[coke]

One of the common failure modes for partcl is that a subclass of a parrot core PMC causes a problem due to code in the PMC that assumes (accidentally) that it will never be subclassed.

So, one way to protect HLL authors is to, for each core PMC test, have a corresponding test file that runs similar/identical tests except with a subclass. (The subclass can be a basic subclass with no overrides.)

It would be nice if the testing framework did this automatically, so that test writers did not have to keep 2 files per PMC in sync whenever tests were added.