Flesh out test for horizontal waypoints
Flesh out test for horizontal waypoints

I wanted to get a few tests in place for the recently added horizontal
waypoint support:

 - having window be the scrollable ancestor
 - verifying that scrolling actually triggers the callbacks

While in the test file, I ended up cleaning out a few unnecessary styles
on the horizontally scrollable parent, reformatting some objects, and
removing a few unused `this.` assignments.

brigade/react-waypoint,brigade/react-waypoint