[9.20] chg: test: Rewrite kasp system test to pytest (4)

These tests do not easily fit in the standard test case framework, so they go into their own suite.
- zsk retired case
- checkds cases
- reload/restart
- inheritance tests

Backport of MR !10278

Merge branch 'backport-matthijs-pytest-rewrite-kasp-system-test-4-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10424