tests: add src/test_phase

Signed-off-by: Jean Privat <jean@pryen.org>
test_phase: provide a simpler way to implement POC of nit tools

Signed-off-by: Jean Privat <jean@pryen.org>
src: new example tool test_test_phase.nit

This module could be shown to first-time contributors (like students)
that need to implement some proof of concept.

Signed-off-by: Jean Privat <jean@pryen.org>
tests: add src/test_test_phase

Signed-off-by: Jean Privat <jean@pryen.org>
Merge: Easy entry point for student

Simplify the entry point for new contributor of the tools.
4th (and last) of the #353 series.

Pull-Request: #353
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>