Tracking issue: test suite

Still to do:
- [x] Create functional tests (call `randombytes` twice, check if output differs)
- [x] Write a mocking test suite (using ~cmocka~ a home-brewed mocking framework)

Platforms:
- [ ] `windows`: use Appveyor (only builds)
- [ ] `linux<3.17` (`/dev/urandom`): [use Travis](https://github.com/dsprenkels/randombytes/blob/master/.travis.yml#L4)
- [ ] `linux>=3.17` (`getrandom`): use Travis?
- [ ] `osx`: [use Travis](https://github.com/dsprenkels/randombytes/blob/master/.travis.yml#L5)
- [ ] `bsd`: **??**
- [ ] `node`: Use Travis (https://github.com/dsprenkels/randombytes/commit/d4a32f8e2b82630459d8bf5c0f30d5d49fdca185)

**Original report:**

Hi there again :^) thanks for sharing this pretty clean and handy implementation. Have you thought of adding a test suite that helps measuring the entropy on each system it is ran? This library may be of help https://github.com/dyne/libdisorder