Add test build in opam file

Beyond the more complete opam file, this would allow to improve CI script (no need to install dependencies manually) and make easier for users to run test themself.

Resources

• Reference blog post : https://opam.ocaml.org/blog/opam-1-2-travisci/
• Example : https://github.com/ocaml/opam-repository/blob/d695c2bf7d8b96d287edf4c0cf36b92f317702ce/packages/git/git.1.8.0/opam#L17

Added CI label

This requires to make sure that oasis file neither require test dependancies (such as alcotest and oUnit) nor try to build test targets (like test.byte or test.native).