Test for <slot> (named and unnamed)

I'm the maintainer of a large private custom element library at work and we we're very excited to see Svelte move to 100% passing tests recently. Several pilot projects had been done with Svelte and finally we adopted it for use in official applications based in part on the fantastic information in this repository.

A few weeks after adoption we discovered Svelte isn't fully compatible with custom elements because of a conflicting "named slot" implementation. We have worked around the issue temporarily, but since slot (or "content projection" in framework nomenclature) is a major function of both custom elements and frameworks I thought a test for would be good to add here.

A related Svelte issue has already been reported by @therealnicksaunders here:

https://github.com/sveltejs/svelte/issues/1689