Bug 1657060 - Disable app update staging in AUS tests. r=mossop

Differential Revision: https://phabricator.services.mozilla.com/D86128

toolkit/mozapps/update/tests/unit_aus_update/xpcshell.ini

@@ -5,6 +5,8 @@
 [DEFAULT]
 tags = appupdate
 head = head_update.js
+prefs =
+  app.update.staging.enabled=false
 support-files =
   ../data/shared.js
   ../data/sharedUpdateXML.js