Add licenses for tests and test data

- Add license headers to source files (years based on commit dates)
  in `src/test` as well as `qa`
- Add `README.md` to `src/test/data` specifying MIT license

Fixes #3848

src/test/transaction_tests.cpp

@@ -1,4 +1,6 @@
-
+// Copyright (c) 2011-2014 The Bitcoin Core developers
+// Distributed under the MIT/X11 software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
 #include "data/tx_invalid.json.h"
 #include "data/tx_valid.json.h"