Allow exit animations to play before step tooltip disappears.

[BrianSipple]

Closes #277

This removes our own manual management of this.el's appearance/presence
in the DOM -- since tippy is handling it already.

This allows the tooltip's exit animation to play, as opposed to having the element disappear instantly before we even call tippy.hide.

[BrianSipple]

This got a bit harrier than I anticipated -- for a few reasons:

As I started testing the step cleanup process to account for the Step.hide changes, I realized that some of our unit test blocks weren't properly tearing down their Tour instance -- so that accounts for some of the structural changes.

I also moved the Step cleanup tests out of Cy and into Mocha to try and keep it as focused as possible. (I continue to have issues with Cy finding elements that "should not exist" because they did exist at some point during the test 😕).

Finally, I refactored done so that it calls destroy on the current step instead of hide, while also separating currentStep.hide outside the concern of Tour._setupActiveTour.