Fix blocked upstream (IP) handling

[DL6ER]

What does this implement/fix?

Queries blocked upstream with a known blocking page IP should not be short-circuited locally as this will cause FTL to reply with the locally defined blocking mode instead of the IP address of the upstream DNS server's blocking page

The tests have been adjusted to test for the new behavior. The now succeeding tests verify it is working as expected. Local testing is possible by specifying the Cisco Umbrella 208.67.222.123, 208.67.220.123 servers as upstream servers and then testing a blocked domain (which is not on your gravity!), e.g., http://www.internetbadguys.com/.

Related issue or feature (if applicable): This is a regression of #2030 and fixes #2175

Pull request in docs with documentation (if applicable): N/A

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