Switch testing framework from MAIN to new UNITTEST network

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

src/test/test_bitcoin.cpp

@@ -31,7 +31,7 @@ struct TestingSetup {
 
     TestingSetup() {
         fPrintToDebugLog = false; // don't want to write to debug.log file
-        SelectParams(CBaseChainParams::MAIN);
+        SelectParams(CBaseChainParams::UNITTEST);
         noui_connect();
 #ifdef ENABLE_WALLET
         bitdb.MakeMock();