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

Introduce module `test_doc` that implements "doc-units".

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 called test-suites.

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>
nitunit: allow test-suite 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>