[Extensions] Divide safety check test with loadTimeData setup

loadTimeData should never be updated in the middle of a test, since it
never changes for the lifetime of the page. It should only be set as the
1st step before anything else is setup.

Thus, we divide a test into three separate test cases that verify the
behavior when the loadTimeData features are disabled or enabled.

This has no change in functionality

Bug: 340606782
Change-Id: Ic31b89a694710b59cbccf79440336d9186906c48
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5552031
Commit-Queue: Emilia Paz <emiliapaz@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1304085}