Merge doc/unit-tests.md into src/test/README.md

Refer to the right file in the top-level README.md.

Having only one file with test documentation saves some confusion about
where things are documented.

README.md

@@ -49,9 +49,10 @@ lots of money.
 
 ### Automated Testing
 
-Developers are strongly encouraged to write [unit tests](/doc/unit-tests.md) for new code, and to
+Developers are strongly encouraged to write [unit tests](src/test/README.md) for new code, and to
 submit new unit tests for old code. Unit tests can be compiled and run
-(assuming they weren't disabled in configure) with: `make check`
+(assuming they weren't disabled in configure) with: `make check`. Further details on running
+and extending unit tests can be found in [/src/test/README.md](/src/test/README.md).
 
 There are also [regression and integration tests](/qa) of the RPC interface, written
 in Python, that are run automatically on the build server.