SignedExchange: Add wpts to test interactions with appcache

We add two tests:
- SXG served by appcache should load (this doesn't yet pass in Chromium)
- SXG should take precedence when there's content in both appcache+sxg-inner-resp

Bug: 940797
Change-Id: Id1463b92f22ab04859ca3f075ca0340225f00694
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1516533
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#642423}
12 files changed