Do a todo: test that callback is called for TestObj#new_callback.
Do a todo: test that callback is called for TestObj#new_callback.

Also, start reformatting the Regress test suite to standard MiniTest::Spec.

jcupitt/gir_ffi,mvz/gir_ffi,jcupitt/gir_ffi,mvz/gir_ffi,mvz/gir_ffi