Make sure to pass the WPT fetch tests

[andreubotella]

There are a number of WPT tests in the fetch folder that deal with multipart/form-data serializing and parsing. Since our serializer seems to match all implementations, it should then be guaranteed that it passes all tests (that don't fail everywhere), but it's best to make sure of it. And the parsing tests will help ground our parser in reality.

Rather than checking if the tests pass by hand, it's probably best to add a reference implementation and make that pass the tests.