chg: test: Convert the digdelv system test from shell to pytest

The digdelv system test was a 2000-line tests.sh script run through the shell-test wrapper. It is now four native pytest modules — tests_dig.py, tests_delv.py, tests_mdig.py and tests_others.py (nslookup, host, nsupdate) — with shared helpers in common.py, repeated checks parametrized, and the yamlget.py helper script replaced by direct PyYAML parsing. PyYAML is now a required test dependency: the +yaml checks fail instead of being skipped when it is missing, and each module is skipped as a whole when its tool is not built.

Merge branch 'mbasti/digdelv-tests-to-py' into 'main'

See merge request isc-projects/bind9!12446