Fix regressions in debian/bin/test-patches

The debian/control-real target in debian/rules no longer includes an 'exit 1'. Change test-patches to stop inverting its result.

If code signing (i.e. signing of the kernel image) is enabled, we no longer build a linux-image-unsigned package directly, and none of the packages that we build will run the linux-image maintainer script hooks. This means that the output of test-patches isn't usefully installable. Change it to disable signing.