Modernize WPT encoding tests that use MessageChannel to transfer

As an example, see `encodeInto.any.js` and the subtest `encodeInto() and a detached output buffer`. T

We won't implement what they're trying to do with `MessageChannel`, but we could offer to update these tests to use the standard way to do it.

If not, we need to check if we have existing coverage or create our own version of the test if needed.