Bug 585291 - FormEngine tests leak [r=mconnor]

Nuke service references on xpcom-shutdown, particularly Svc.Form (nsIFormHistory2) which doesn't clean up after itself as it was only used by nsIFormAutComplete so far.

services/sync/modules/util.js

@@ -918,3 +918,8 @@ Svc.Obs = Observers;
 let Str = {};
 ["errors", "sync"]
   .forEach(function(lazy) Utils.lazy2(Str, lazy, Utils.lazyStrings(lazy)));
+
+Svc.Obs.add("xpcom-shutdown", function () {
+  for (let name in Svc)
+    delete Svc[name];
+});