Add respdiff test
This is a port of respdiff test from the bind-qa repo to GitLab CI.
The test is mostly added as-is, modulo shellcheck fixes to the respdiff.sh runner and a summary on query answers disagreements in the runner.
For the package installation part to be dropped from the CI job, images!22 (merged) needs to be merged first.
respdiff job: https://gitlab.isc.org/isc-projects/bind9/-/jobs/688192.