tests: only run gpgsign tests if gpg is present and executable
tests: only run gpgsign tests if gpg is present and executable

This allows the full test suite to run in Debian/Ubuntu autopkgtest with
only the Depends: installed, and nothing from Recommends:.

f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver