Refactor MakeSingleRedirectAndWait() in PrefetchServiceTest

And introduce `Disconnect()` helper.

This is mostly mechanical and doesn't change the test behavior, except
for `IneligibleSameSiteCrossOriginRequiresProxyRedirect` where
`redirect_info.new_url` and `CreateURLResponseHeadForPrefetch()`'s URL
are different. Based on the test comment, this CL uses the current
`redirect_info.new_url` for both of them after this CL, as the test is
about same-site cross-origin URLs, and the difference between these
doesn't seem to affect the test behavior meaningfully here.

Bug: 400761083
Change-Id: I9b9465ed7600525fef28cedc3ed05ba590dae303
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6859046
Reviewed-by: Ken Okada <kenoss@chromium.org>
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1503571}