Merge: nitunit: compile test-suite only once

Thanks to this PR, all the test-cases in a test-suite are compiled in one file.
Nitunit then uses an argument to select which test-case to execute.

This allows a BIG gain of performances for large test-suites.

Special thanks @privat for the idea.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #996
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>