Implement system tests that check Cascade's zone output

[bal-e]

Cascade can serve zone data (whether for review or as the published output) in multiple ways -- these need to be checked for consistency.

• Test that a SOA query matches the SOA record in the AXFR output
• Test that all unsigned records are present in signed zones' AXFR outputs

Tests that cannot be implemented yet:

• Test that every record in AXFR output matches its individual DNS query
• Test that IXFR output (where diff-based or absolute) is consistent with the overall AXFR
• Test that IXFR returns a diff only if it is smaller than the absolute AXFR
• Test that zonefile output matches AXFR output