kasp: Add test case for migrating KSK/ZSK to CSK

kasp: Add test case for migrating KSK/ZSK to CSK

Add a test case to cover #3679 where a user migrates from a KSK/ZSK
split using auto-dnssec maintain, to the default dnssec-policy (CSK).

The test actually does not use the default dnssec-policy, but it does
use one that has the same keys clause. For testing convenience, we use
the same propagation time values as other test cases that migrate to
dnssec-policy with mismatching existing key set.