tests: conditionally disable tests that can't work with apksigner
tests: conditionally disable tests that can't work with apksigner

apksigner treats MD5 signatures as valid, fdroid does not.

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