Recipe: OCaml test suite

[hobovsky]

Use material collected when working on https://github.com/codewars/ocaml/issues/1 and https://github.com/codewars/ocaml/pull/2 and prepare a guide similar to other language specific authoring guides, like for example the one for Javascript.

Example kumite: https://www.codewars.com/kumite/620a7361d746e5000f36a021/

Features:

• TestLists, with and without labels
• TestCases, with and without labels
• Nested tests
• Nested labels
• Custom assertion messages and printers

[Glinat]

Here is an example of a test suite using the optional pp_diff argument of assert_equal : https://www.codewars.com/kumite/62ff92cb211df8001c4b80c4

This argument is used to show the differences between the expected and actual answers in a custom way.

[Glinat]

When working with lists, ~pp_diff and ~printer can be automatically generated with the OUnitDiff module, as demonstrated in its documentation.