Add tests for two classes with methods

[adetaylor]

We should add an integration test (to integration_tests.rs) which has two structs or classes, each with methods.

If autocxx is behaving right, it will generate the receivers as self: &StructName and cxx will handle it right. If autocxx gets it wrong, they will both be generated as simply self and cxx will barf.

Related comment: dtolnay/cxx#370 (comment)