patchtest: shorten patch signed-off-by test output

patchtest: shorten patch signed-off-by test output

The patch Signed-off-by test's output line is excessively long, and can
trigger a failsafe in the patchtest automated service's email content
generation. Shorten the output by reducing redundant phrasing and using
os.path.basename to get only the failing patch's name, not the entire
path, as the submitter should have a good idea of where it is located
regardless.

Signed-off-by: Trevor Gamblin <tgamblin@baylibre.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit cc7546ded87dd44a988d7a23f1d7645094b5cdd4)
Signed-off-by: Steve Sakoman <steve@sakoman.com>