Put .rdoc at the end of the files named RUNNING_UNIT_TESTS

Now RUNNING_UNIT_TESTS look better. See how they look:

* [rails / activerecord / RUNNING_UNIT_TESTS.rdoc](https://github.com/yuki24/rails/blob/c8a25386d39d49d9ce5135a413f19c416456299e/activerecord/RUNNING_UNIT_TESTS.rdoc)
* [rails / actionpack / RUNNING_UNIT_TESTS.rdoc](https://github.com/yuki24/rails/blob/c8a25386d39d49d9ce5135a413f19c416456299e/actionpack/RUNNING_UNIT_TESTS.rdoc)