Use @prefix for prefix mapping in rdfa1.1 test-case output, and use xmls:foo for rdfa1.0 output.

There are a number of existing test cases in the test suite that generate HTML5 output that uses xmlns:foo for prefix mappings, which isn't actually valid in HTML5. This PR changes the test-case sources to use JSON to express the prefix mappings, and changes the backend generation of the serialized mappings from that JSON to be conditional:

* if the output version is RDFa 1.0, then the backend uses the JSON to output@prefix attributes for the mappings
* if the output version is RDFa 1.1, then the backend used the JSON to output xmlns:foo instances for the mappings