Tests refactoring checklist

Initially, I created a [project](https://github.com/joshwcomeau/react-flip-move/projects/2), but then I realised that it's not commentable so maybe an umbrella issue with checklist would be more useful, so I duplicate it as such.

@joshwcomeau has [mentioned](https://github.com/joshwcomeau/react-flip-move/pull/151#issuecomment-301244708) that he would like to have some improvements. So I decided to make some kind of checklist for what has to be done. Feel free to add anything that you think would be nice to do.

Definitely needed:
- [x] Isolate the tests from each other, i.e. make all the setup before each test so that the state is reset #166
- [ ] Add tests for appear / enter / leave animations
- [x] Do something with tests stability (see false positives like that: https://travis-ci.org/joshwcomeau/react-flip-move/jobs/236306660) #175
- [x] Add tests for runtime props conversion and checks #174
- [ ] Use sandboxed sinon for easier resetting/restoring

Maybe needed:
- [x] ~~Add simple unit tests for helpers~~ — not really needed, they are fully covered by other tests
- [x] Use some code coverage tool like [istanbul](https://github.com/gotwarlost/istanbul) #172
- [x] Use [enzyme](https://github.com/airbnb/enzyme) for easier react output manipulation #169
- [x] ~~Use sinon fake timers instead of `setTimeout`~~

This is an umbrella checklist rather than a single implementation task; it mentions the project board, PR #151, and completed work in issues #166, #175, #174, #172, and #169. Start by reviewing the remaining unchecked items and the existing test setup. Done means the selected test-refactoring item is completed and its checklist entry can be marked accordingly.

Written by the indexing model from the issue text.