fix: dev: Update dnssec validation test to match new behavior

Some of the tests in `dnssec/tests_validation.py` worked by iterating through the response message looking for failure conditions, such as excessively high TTL values. In some cases, previous changes caused additional data not to be returned. Since there was nothing to iterate, the tests still "passed".

Tests that don't make sense anymore have been removed. Other tests that iterate through responses have been updated with checks to ensure that the responses actually do contain data.

Merge branch 'each-cleanup-validation-test' into 'main'

See merge request isc-projects/bind9!12269