Switch testing framework from MAIN to new UNITTEST network

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

src/pow.cpp

@@ -81,6 +81,10 @@ bool CheckProofOfWork(uint256 hash, unsigned int nBits)
     bool fNegative;
     bool fOverflow;
     uint256 bnTarget;
+
+    if (Params().SkipProofOfWorkCheck())
+       return true;
+
     bnTarget.SetCompact(nBits, &fNegative, &fOverflow);
 
     // Check range