setting up a test that actually subclasses BaseModel
setting up a test that actually subclasses BaseModel

Summary:
- Add a separate instr test that subclasses `BaseModel`
- Testing a roundtrip from Java obj to FlatBuffer and back
- Type erasure apparently doesn't work here yet.

Reviewed By: justinjhendrick

Differential Revision: D5652725

fbshipit-source-id: eedd025b60613e4ba28cd804ff4789361cc634be

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