replace js scrollspy test for default behavior

The "should switch active class on scroll" test looked incorrect to me so I replaced it. In this case `ok($topbar.find('.active'))` was always truthy.

I'm not very experienced with qunit and used promises to make the tests pass, I'm not sure if that is a good practice. I also would have preferred to make separate tests with a setup but qunit doesn't support "sub modules" yet.

I wrote more tests and will submit them if this one is approved (if code style is ok etc).