Make digdelv test work in different network envs

Make digdelv test work in different network envs

When a target server is unreachable, the varying network conditions may
cause different ICMP message (or no message). The host unreachable
message was discovered when attempting to run the test locally while
connected to a VPN network which handles all traffic.

Extend the dig output check with "host unreachable" message to avoid a
false negative test result in certain network environments.

(cherry picked from commit 1e7d832342aa2aaaeb8ef1710ec095dfcade4c6d)