Fix sandbox top navigation tests using external URLs.

The sandbox top navigation WPTs currently attempt a top-level navigation
to 'https://google.com'. Not only is using external URLs bad practice,
Firefox's test harness code refuses to make requests to remote URLs.
This causes these tests to crash in their trybots.

This CL updates the tests to point to the WPT-supported URL
'http://{{hosts[alt][www2]}}:{{ports[http][0]}}'. This is chosen so the
URL will be cross-site to the URLs the main page and the iframes will
be navigated to in the various tests.

See this comment on the WPT GitHub repo:
https://github.com/web-platform-tests/wpt/pull/35697#issuecomment-1970026245

Change-Id: I8b893d5c61c91d76acf15e47c4fe42c28a09cca5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5357673
Reviewed-by: Garrett Tanzer <gtanzer@chromium.org>
Commit-Queue: Liam Brady <lbrady@google.com>
Cr-Commit-Position: refs/heads/main@{#1270985}