This works:

```javascript
    suite('with a contact with no picture', function() {
      setup(function() {
        subject.photo = null;
        MockCallScreen.mSetCallerContactImageCalled = false;
        mockCall._connect();
      });

      test('wallpaper displaying', function() {
        assert.isFalse(MockCallScreen.mSetCallerContactImageCalled);
        assert.isTrue(MockCallScreen.mSetDefaultContactImageCalled);
      });
```