Preserve tx version=1 for certain tests

Without this change, the tests would be affected by default
tx version increases.

src/test/hash_tests.cpp

@@ -124,7 +124,11 @@ BOOST_AUTO_TEST_CASE(siphash)
     }
 
     CHashWriter ss(SER_DISK, CLIENT_VERSION);
-    ss << CTransaction();
+    CMutableTransaction tx;
+    // Note these tests were originally written with tx.nVersion=1
+    // and the test would be affected by default tx version bumps if not fixed.
+    tx.nVersion = 1;
+    ss << tx;
     BOOST_CHECK_EQUAL(SipHashUint256(1, 2, ss.GetHash()), 0x79751e980c2a0a35ULL);
 }