Resolver prefetch eligibility edge case bug

According to ARM, the prefetch "eligibility" parameter is the smallest original TTL value that is accepted for a record to be eligible for prefetching, but in fact, when the original TTL value is equal to the eligibility value, the record is not treated as eligible for prefetching.

For example, with prefetch 4 10; configuration in tests/system/resolver/ns5 instance, prefetch doesn't work for the following records in tests/system/resolver/ns4:

fetchall  10              TXT     A short ttl
fetchall  10              A     1.2.3.4
fetchall  10              AAAA    ::1

Although the test passes because it fails to check that prefetch occurred.