Actually disable tests for global PB if not supported a=@gozala

(cherry picked from commit 81dbd8fd9e07e347696404a9e0e7761f5beac005) a=@erikvold

Conflicts:

	test/addons/private-browsing-supported/main.js
Merge pull request #895 from erikvold/stabilization

Bug 839746 Actually disable tests for global PB if not supported a=@erikvold