debian: Merge the testsuite execution into the build-arch target

Use a build stamp file so that we do not invoke it from a binary
target. The latter is going to be run as root possibly via fakeroot,
and as GnuPG is set-uid-root on non-Linux systems, it fails there.
debian: Mark targets as .PHONY
debian: Explicitly Build-Depend on gnupg for the testsuite
debsig-verify 0.12
Disable all current GnuPG warnings

These do not concern us, as we only use it for verification purposes,
so we should not be handling sensitive material anyway.

This fixes failures in the testsuite on GNU/Hurd due to unexpected
output in stderr.
debsig-verify 0.13