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/data/README.md

@@ -0,0 +1,14 @@
+Description
+------------
+
+This directory contains data-driven tests for various aspects of Bitcoin.
+
+License
+--------
+
+The data files in this directory are
+
+    Copyright (c) 2012-2014 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.
+