nitunit: split module into one executable and a group named `testing`.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitunit: factorize test_dir processing into test_base.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitunit: Allow testing from external files.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitunit: Allow external test file skeleton generation.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitunit: add README for group `testing` and redirect from nitunit doc.
nitunit: update tests.