angularjs http service unit testing

I am trying to test a simple service for learning purposes..However; I can't figure out how it must be done:

service:

.factory('myService', function($http) {
  var myService = {
    async: function() {
      var promise = $http.get('test.json').then(function (response)
{
        return response.data;
      });
      return promise;
    }
  };
  return myService;
});

controller:

myService.async().then(function(d) {
    $scope.data = d;
    $scope.e = $scope.data.txt;
  });

test:

'use strict';
describe("myService", function(){

    beforeEach(module("testingExpApp"));

    var myService,
        $httpBackend;

    beforeEach(inject(function(myService, _$httpBackend_){
        myService = myService;
        $httpBackend = _$httpBackend_;
    }));

    describe("get", function(){

        it('should return test.json data', function () {
        var url = "../mock/test.json";
        var x = $httpBackend.expectGET(url).respond(200, 'txt from json');

         // flush response
        $httpBackend.flush();
        expect(x).toBe('txt from json');

        });

    });

});

I get 'no pending request to flush!'

I just want to test that myservice.get() get the test.json file data..I have tried everything but can't get it working..

Any tips?

Thanks a lot in advance!