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/DoS_tests.cpp

@@ -1,3 +1,7 @@
+// 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.
+
 //
 // Unit tests for denial-of-service detection/prevention code
 //