Update serializer tests to give expectations as per the HTML fragment serialisation algorithm

[gsnedders]

Because our current expectations are crazy. (See #30.)

We should have expectations with attributes both in source order and in alphabetical order (the spec makes the order undefined, but we obviously can't test that).

[gsnedders]

We should also update the input format to be something more sane. Probably just make it HTML, and allow fragments.

[gsnedders]

wontfix given #30 removed them

[gsnedders]

wait, no, that's untrue