Update Unsafe Browser test to use about:support instead of about:addons
This is about the
the Unsafe Browser has no add-ons installed step.
about:addons got broken in Tor Browser 4.5 when Torbutton isn’t
enbaled (as in the Unsafe Browser). See #9031 (comment 78691).
Feature Branch: test/9306-fix-unsafe-browser-addons-check