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.

doc/README.md

@@ -53,7 +53,6 @@ The Bitcoin repo's [root README](/README.md) contains relevant information on th
 - [Source Code Documentation (External Link)](https://dev.visucore.com/bitcoin/doxygen/)
 - [Translation Process](translation_process.md)
 - [Translation Strings Policy](translation_strings_policy.md)
-- [Unit Tests](unit-tests.md)
 - [Travis CI](travis-ci.md)
 - [Unauthenticated REST Interface](REST-interface.md)
 - [Shared Libraries](shared-libraries.md)