Bug 922981 - Split marDownload.js into individual tests. r=bbondy

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rename : toolkit/mozapps/update/tests/unit_aus_update/downloadMar.js => toolkit/mozapps/update/tests/unit_aus_update/downloadAndHashCheckMar.js
rename : toolkit/mozapps/update/tests/unit_aus_update/downloadMar.js => toolkit/mozapps/update/tests/unit_aus_update/downloadInterruptedRecovery.js