Replace XapTest with a generated fixture.
Replace XapTest with a generated fixture.

This version has a much better coverage of the metadata and features half a
dozen of modification tests.

archrival/taglib-sharp,Clancey/taglib-sharp,Clancey/taglib-sharp,CamargoR/taglib-sharp,punker76/taglib-sharp,archrival/taglib-sharp,CamargoR/taglib-sharp,hwahrmann/taglib-sharp,Clancey/taglib-sharp,punker76/taglib-sharp,mono/taglib-sharp,hwahrmann/taglib-sharp