new: test: Test migrating to dnsec-policy

Various test cases where legacy keys exist and are used to migrate to 'dnssec-policy'. These once existed as shell script but were removed because 'auto-dnssec' was removed. But the test cases are still useful if one wants to migrate to 'dnssec-policy' with such legacy keys, so it would be good to resurrect these tests.

Closes #3769

Merge branch '3769-migrate-zsk-ksk-plit' into 'main'

See merge request isc-projects/bind9!10787