Don't allow web worker grammars to be registered

Grammars are already loaded at the beginning of the
specs synchronously and should not be overridden
when testing the background load task.

spec/app/atom-spec.coffee

@@ -60,6 +60,10 @@ describe "the `atom` global", ->
   describe ".loadPackages()", ->
     beforeEach ->
       window.rootView = new RootView
+      spyOn(syntax, 'addGrammar')
+
+    afterEach ->
+      delete window.rootView
 
     it "terminates the worker when all packages have been loaded", ->
       spyOn(Worker.prototype, 'terminate').andCallThrough()