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.

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

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