docs(guide/Unit Testing): change $scope = {} to $scope = $rootScope.$new()

{} will just create an empty object. This will break if the module uses for example $watch or others.
While it's not necessary for this example, it's good general practice.

Closes #16315

Author: lisapfisterer

docs/content/guide/unit-testing.ngdoc

@@ -149,16 +149,17 @@ for instantiating controllers.
 describe('PasswordController', function() {
   beforeEach(module('app'));
 
-  var $controller;
+  var $controller, $rootScope;
 
-  beforeEach(inject(function(_$controller_){
+  beforeEach(inject(function(_$controller_, _$rootScope_){
     // The injector unwraps the underscores (_) from around the parameter names when matching
     $controller = _$controller_;
+    $rootScope = _$rootScope_;
   }));
 
   describe('$scope.grade', function() {
     it('sets the strength to "strong" if the password length is >8 chars', function() {
-      var $scope = {};
+      var $scope = $rootScope.$new();
       var controller = $controller('PasswordController', { $scope: $scope });
       $scope.password = 'longerthaneightchars';
       $scope.grade();