Add kasp tests for offline keys

Add kasp tests for offline keys

Add a test for default.kasp that if we remove the private key file,
no successor key is created for it. We need to update the kasp script
to deal with a missing private key. If this is the case, skip checks
for private key files.

Add a test with a zone for which the private key of the ZSK is missing.

Add a test with a zone for which the private key of the KSK is missing.

(cherry picked from commit 4a8ad0a77f7f8baa0fda4d30760b6fbd2e617845)