Test that re-exported `types` auxiliary types are correct

[tcharding]

Given RPC method foobar returning type FooBar with conversion error FooBarError.

Our integration testing strategy verifies that FooBar is re-exported correctly. If FooBar has a field with type FooBarAuxType then we do not currently verify that it is correctly re-exported - i would be nice to come up with a way to prove we got it right. Extremely high chance I made mistakes in this area. Not super likely downstream will notice.