Add exercises tests

[cuihtlauac]

Add a self-testing mechanism to the exercises. A mean for the user to check its implementation without reading the solution.

As a follow-up, those tests should be part of ocaml.org tests to check the validity of the proposed solutions.

[sabine]

It would be really nice if every exercise had a test-suite. And then integrate that with the playground somehow:

1. there's a button "open in playground" which leads to a playground that contains the problem statement and some skeleton code to fill in.
2. problem's playground has a special button "check my solution" that runs the problem's test suite

Something like that, right?

[cuihtlauac]

Definitely, that would be so cool

[amarachigoodness74]

@sabine Can I try working on this? @SaySayo @cuihtlauac @tmattio

[SaySayo]

You can go ahead @amarachigoodness74.

[0xrotense]

Hello @SaySayo, Can i also work on this together with @amarachigoodness74 ?