Add DOM fixture page for Fragment Ref (#32527)

This adds a page to the DOM fixture to test Fragment Refs. The first
test case is for `addEventListener`/`removeEventListener`.

Setting `enableFragmentRefs` to `__EXPERIMENTAL__` and building is
required to run the fixture.

<img width="872" alt="Screenshot 2025-03-05 at 12 58 57 PM"
src="https://github.com/user-attachments/assets/fee498b7-fd96-4178-9e82-c46d4cb55c9b"
/>