Bug 817447 - Create download manager unit tests that do not rely on the global...

Bug 817447 - Create download manager unit tests that do not rely on the global private browsing service. r=ehsan

DONTBUILD because this is NPOTB for global PB builds

--HG--
rename : toolkit/components/downloads/test/unit/test_private_resume.js => toolkit/components/downloads/test/unit/test_private_resume_global.js
rename : toolkit/components/downloads/test/unit/test_privatebrowsing_cancel.js => toolkit/components/downloads/test/unit/test_privatebrowsing_cancel_global.js
rename : toolkit/components/downloads/test/unit/test_privatebrowsing.js => toolkit/components/downloads/test/unit/test_privatebrowsing_global.js