[Extensions] Add UI test for userScripts API toggles.

Before this we had no UI test for the dev mode and per-extension toggles
that control chrome.userScripts API availability.

After this change we have a test that confirms:
  1) chrome.userScripts API is not available by default
  2) flipping the toggles to enabled makes the chrome.userScripts API
     available and allows script inject
  3) flipping the toggles to disabled disallows the above

Bug: 390138269
Change-Id: I90eac0701a27b3b244ff1a08f04bee309873a2ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6420151
Auto-Submit: Justin Lulejian <jlulejian@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Devlin Cronin <rdevlin.cronin@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1443816}