ptest-cargo: fix tests output format for testimage

ptest-cargo: fix tests output format for testimage

In testimage, the ptest-runner output parser expects test results to follow
a specific format,with lines beginning with PASS:, FAIL:, or SKIP:. ptest-cargo,
currently, does not emit any of those lines and the parser treats the test
section as having no results, causing a test failure with :

AssertionError:
ptests which had no test results:
['<package>']

This patch ensures that the recipes using ptest-cargo class explicitly emits
PASS: or FAIL: lines, making the results compatible with the test parser and
preventing test failures.

Signed-off-by: Ines KCHELFI <ines.kchelfi@smile.fr>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>