Moved the fetch parts of the priority hints tests

Moved the fetch parts of the priority hints tests into fetch as we're
getting ready to land the spec changes.

This also adds tests that successful use of the priority requestinit
attribute works as expected and that the internal-only attributes
(blocking, priority and internalpriority) are not exposed on Request.

Bug: 1301942
Change-Id: I311fba04587d1eb50b89c0f9303ac628d86a9529
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4097472
Commit-Queue: Patrick Meenan <pmeenan@chromium.org>
Reviewed-by: Domenic Denicola <domenic@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1104084}