Add tests that two-way binding facilities work with custom elements

This [Svelte issue](https://github.com/sveltejs/svelte/issues/4838) came across my desk today and I think we should have a test that covers this kind of thing. The test would check that if a framework supports a 2-way binding syntax that works against some built-in elements, does it also work with custom elements with any (attribute|property x event) pairing.