Add system test for forward only grafted zone with synth-from-dnssec

Add system test for forward only grafted zone with synth-from-dnssec

We are grafting on an unsigned zone "example.internal" where the higher
zone (".") is signed and would otherwise cause named to synthesise a
NXDOMAIN for example.internal.  We prime the cache by performing a
lookup for "internal" and then lookup "example.internal".

(cherry picked from commit 8af5d0ad68ca939d98dd57706b534c58741e94a8)