test($anchorScroll): fix tests in order to reliably pass on Firefox

There was an timing issue (that seemed to only affect Firefox) due to
`scroll()` being wrapped in `$evalAsync()` (this is necessary to ensure
that Firefox does not overwrite the `yOffset` when navigating to a new
page).
This commit wraps the some expectations in `$evalAsync()` in order to
evaluate them after `scroll()` has been called.