Refactor setup for functional test cases.

This includes several elements to improve handling of functional test cases.

Subclasses should now implement a method which returns the path to the WSDL. This means they no longer have to override the setup function to set the path as a property. This also means we can move code generation has been moved to the setup function.

Move WSDLs and XSDs to subdirectories under test fixtures to get a better overview. This also prepares for inclusion of many WSDLs from https://github.com/spox/soap4r-spox/tree/master/test/wsdl which could help us test more edge cases.