Bug 1549859 - set the app.update.BITS.inTrialGroup pref in app update xpcshell tests. r=bytesized

This causes downloadInterruptedNoRecovery.js to intermittently fail on comm-central since BITS ends up enabled.

Differential Revision: https://phabricator.services.mozilla.com/D30284

--HG--
extra : moz-landing-system : lando

toolkit/mozapps/update/tests/data/xpcshellUtilsAUS.js

@@ -843,6 +843,8 @@ function setupTestCommon(aAppUpdateAutoEnabled = false, aAllowBits = false) {
 
   setAppUpdateAutoSync(aAppUpdateAutoEnabled);
   Services.prefs.setBoolPref(PREF_APP_UPDATE_BITS_ENABLED, aAllowBits);
+  // Set this preference so the trial doesn't override the decision.
+  Services.prefs.setBoolPref("app.update.BITS.inTrialGroup", aAllowBits);
 
   debugDump("finish - general test setup");
   return true;