Remove serializer tests

[gsnedders]

These test specific serializer options, and expect character-for-character identical serializations: these belong in individual projects implementing the serializer, not being shared, as they are obviously not going to be universal. See html5lib/html5lib-python#95 for something that gratuitously fails them despite having perfectly sensible output (arguably better than what the tests expect)!

[gsnedders]

We won't. We'll make them sane. See #58.