- 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
Unit tests would fail if compiled with -DDEBUG_LOCKORDER (AssertLockHeld()
would fail; AssertLockHeld() relies on the DEBUG_LOCKORDER code to keep
track of locks held).
Fixed by LOCK'ing the wallet mutex in the unit tests that manipulate the
wallet.