Switch testing framework from MAIN to new UNITTEST network

UNITTEST inherites from MAIN but allows synamically changing its parameters using the ModifiableParams() interface

src/checkpoints.cpp

@@ -88,6 +88,8 @@ namespace Checkpoints {
             return dataTestnet;
         else if (Params().NetworkID() == CBaseChainParams::MAIN)
             return data;
+        else if (Params().NetworkID() == CBaseChainParams::UNITTEST) // UnitTest share the same checkpoints as MAIN
+            return data;
         else
             return dataRegtest;
     }