Change Angular mock value

I have an unit test that look like this:

describe("myDirective Test",function(){
    beforeEach(module('app'));

    beforeEach(function() {
        module(function($provide) {
          $provide.constant('Config', {"showmenu":true});
        });
    });

    it('should add showmenu attribute',inject(function($compile,$rootScope){
        var element = $compile('<div my-directive></div>')($rootScope);
        expect(element.attr('showmenu')).not.toBe(undefined);
    }));

    it('should NOT add showmenu attribute',inject(function($compile,$rootScope){
        var element = $compile('<div my-directive></div>')($rootScope);
        expect(element.attr('showmenu')).toBe(undefined);
    }));
});

How can I change the showmenu value, so that it is false in the should NOT add showmenu attribute test?