tests: Add nested struct tests for transformer

Add tests for nested structs as they pass through the transformer.

https://bugzilla.gnome.org/show_bug.cgi?id=581525
tests: Add nested union unittests

Add a sub-set of tests found in struct testing as only basic validation is
needed due to the shared code paths.

https://bugzilla.gnome.org/show_bug.cgi?id=581525
tests: Add transformer tests for callback typedefs

Add a basic transformer unittest for callback typedefs.

https://bugzilla.gnome.org/show_bug.cgi?id=581525