Web Platform Tests for Speculation Rules tags.

Explainer:
https://github.com/WICG/nav-speculation/blob/main/speculation-rules-tags.md

Spec:
https://wicg.github.io/nav-speculation/speculation-rules.html#collect-tags-for-matching-speculative-load-candidates
https://wicg.github.io/nav-speculation/prefetch.html#sec-speculation-tags-header
