Fixed all common and LMS tests.

The tests were failing because XMLDescriptor adds in some attributes
to _model_data, such as `xml_attributes`, that aren't necessary. The
solution is to handle all XML parsing in VideoDescriptor. There's
still one test failing in CMS, which has to do with metadata being
saved. I'm still working out how to update it in such a way that it
doesn't fail, but still tests something meaningful.