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>