I'd really prefer to keep CSS selectors out of the test suite so that the test logic is clearer.  The pattern I've used other places is to create a getter in the view for the button's status.  That getter can then access the button in the DOM to check whether the button is enabled or disabled.

Something like this: https://github.com/edx/edx-ora2/blob/master/apps/openassessment/xblock/static/js/spec/oa_response.js#L187