Test suite failures

[peterzen]

Tests that rely on valid RRSIGs currently fail as the signatures in some of the zone files have expired, breaking validation tests. Ideally, the RRSIGs used in the test cases that do not cover code related to signature validity periods (e.g. TestLookupInvalidDsDigest, TestStrictNSQuery) should have period long in the future.

New test cases should be added to cover the code branch returningErrRrsigValidityPeriod.