Modernize WPT encoding tests that use XHR or ensure we have coverage

Some encoding tests, like `replacement-encodings.any.js` and `unsupported-encodings.any.js` use XHR to a data URL just a means of testing behaviour of encoding and decoding.

We have a couple of options available to us:

1. Ask WPT if they'll consider upgrading these tests to use fetch; create a PR if amenable.
2. Otherwise, ensure we have coverage either with existing tests or creating our own version of these tests.