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-note_19.

Feature Branch: test/9306-fix-unsafe-browser-addons-check

Related issues

Related to #9031 (closed)
Related to #9305 (closed)

Original created by @anonym on 9306 (Redmine)

Edited Nov 22, 2019 by intrigeri

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