Specs: add spec for "when I press enter, the component behaves as expected"

[jackkoppa]

Per this comment: #71 (comment)

There are a couple things we'll want to test. Right now, SelectMulti.spec.ts only checks for behavior when clicking options. Let's add some initial specs for pressing enter, and lets add them to both SelectMulti.spec.ts and SelectSingle.spec.ts. Things like:

1. When I press enter on an option, it's selected.
2. When I press enter on an option, it emits the correct event(s)
3. (multi) When I press enter on a selected option, it's unselected
4. (multi) When I press enter not on an option, no event is emitted/nothing happens
5. (multi) When I press enter while on a selected pill, it's unselected