Switch testing framework from MAIN to new UNITTEST network

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

src/main.cpp

@@ -2304,6 +2304,7 @@ bool AcceptBlockHeader(CBlockHeader& block, CValidationState& state, CBlockIndex
         nHeight = pindexPrev->nHeight+1;
 
         // Check proof of work
+        if (!Params().SkipProofOfWorkCheck())
         if (block.nBits != GetNextWorkRequired(pindexPrev, &block))
             return state.DoS(100, error("AcceptBlock() : incorrect proof of work"),
                              REJECT_INVALID, "bad-diffbits");