Add test fixtures and integrate them

It'd be great to be able to add html files for both input and expected output, and have them automatically picked up by the test suite.

We could have a directory structure similar to:

```
test/
fixture-tests.js
fixtures/
unordered-list/
input.html
expected.html
some-other-test/
input.html
expected.html
```

And have `test-fixtures` `glob` for `input.html` to use as the test inputs, and acquire the expected output by grabbing `expected.html` from the same directory and comparing them.

I'm definitely open to other approaches on how to best handle this.