Avoid a race in test code between processing the first query and the priming code.

Avoid a race in test code between processing the first query and the priming code.

It can happen the root updating houskeeping code has not run yet when
processing the first query.  In that case, we do not have root DNSKEY
records in cache.  When validating the answer, rec will then go out and
fetch those, so no real issue, apart form the fact that it interferes
with our expected outgoing protobuf messages.

Same make sure the root DNSKEY records are cached before doing the actual
test query.