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/miner_tests.cpp

@@ -253,6 +253,7 @@ BOOST_AUTO_TEST_CASE(CreateNewBlock_validity)
 
     chainActive.Tip()->nHeight--;
     SetMockTime(0);
+    mempool.clear();
 
     BOOST_FOREACH(CTransaction *tx, txFirst)
         delete tx;