[NTP XDTR] Simplify tests

Move mock calls to shared location. Split up tests so they each test
a one thing.

Bug: 328235012
Change-Id: Iad87b5571224ecf221cc811a18b3ca3fdb48f9ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5385553


Reviewed-by: Roman Arora <romanarora@chromium.org>
Commit-Queue: Marlon Facey <mfacey@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1279180}