testing: Install packages like the FIPS-enabled OpenSSL from a custom apt repo

testing/testing.conf

@@ -53,6 +53,9 @@ fi
 : ${BASEIMGARCH=amd64}
 : ${BASEIMG=$IMGDIR/debian-$BASEIMGSUITE-$BASEIMGARCH.$IMGEXT}
 : ${BASEIMGMIRROR=http://http.debian.net/debian}
+: ${BASEIMGEXTREPOHOST=download.strongswan.org}
+: ${BASEIMGEXTKEY=https://$BASEIMGEXTREPOHOST/testing/repos/strongswan-testing.gpg.key}
+: ${BASEIMGEXTREPO=https://$BASEIMGEXTREPOHOST/testing/repos/apt/debian}
 
 # Directory shared between host and guests
 : ${SHAREDDIR=$BUILDDIR/shared/$BASEIMGSUITE}