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.
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