[test] Introduce test logic to query DNS seeds

This commit introduces a DNS seed to the regest chain params in order to add
coverage to the DNS querying logic.

The first test checks that we do not query DNS seeds if we are able to
succesfully connect to 2 outbound connections. Since we participate in ADDR
relay with those connections, including sending a GETADDR message during the
VERSION handshake, querying the DNS seeds is unnecessary.

Co-authored-by: Martin Zumsande <mzumsande@gmail.com>

src/chainparams.cpp

@@ -435,7 +435,8 @@ public:
         assert(genesis.hashMerkleRoot == uint256S("0x4a5e1e4baab89f3a32518a88c31bc87f618f76673e2cc77ab2127b7afdeda33b"));
 
         vFixedSeeds.clear(); //!< Regtest mode doesn't have any fixed seeds.
-        vSeeds.clear();      //!< Regtest mode doesn't have any DNS seeds.
+        vSeeds.clear();
+        vSeeds.emplace_back("dummySeed.invalid.");
 
         fDefaultConsistencyChecks = true;
         fRequireStandard = true;