Trivial: Add running tests section to HACKING.rst
The HACKING.rst testing section is added to point out
that we use stestr now instead of testr.
@@ -17,3 +17,22 @@ For every new feature, unit tests should be created that both test and
bug that had no unit test, a new passing unit test should be added. If a
submitted bug fix does have a unit test, be sure to add a new one that fails
without the patch and passes with the patch.
+
+Running Tests
+-------------
+The testing system is based on a combination of tox and stestr. The canonical
+approach to running tests is to simply run the command ``tox``. This will
+create virtual environments, populate them with dependencies and run all of
+the tests that OpenStack CI systems run. Behind the scenes, tox is running
+``stestr run``, but is set up such that you can supply any additional
+stestr arguments that are needed to tox. For example, you can run:
+``tox -- --analyze-isolation`` to cause tox to tell stestr to add
+--analyze-isolation to its argument list.
+
+It is also possible to run the tests inside of a virtual environment
+you have created, or it is possible that you have all of the dependencies
+installed locally already. In this case, you can interact with the stestr
+command directly. Running ``stestr run`` will run the entire test suite.
+``stestr run --concurrency=1`` will run tests serially (by default, stestr runs
+tests in parallel). More information about stestr can be found at:
+http://stestr.readthedocs.io/
