ci: Try debian testing for 'libzypp' backend

Dependency libzypp found: NO found 17.25.7 but need: '>=17.31.0'

verion 17.31.0 was released on Jul 19, 2022
verion 17.25.7 was released on Feb 17, 2021

version in debian testing: 17.35.9-1

tests/ci/Dockerfile-debian

@@ -1,4 +1,4 @@
-FROM debian:bookworm
+FROM debian:testing
 
 # allow fetching source packages
 RUN echo "deb-src http://deb.debian.org/debian/ bookworm main" >> /etc/apt/sources.list