Trivial: Update HACKING.rst with stestr

The HACKING.rst testing section is updated a bit to point out
that we use stestr now instead of testr.

Change-Id: Ia4417994def85c989df315ebc17f5a0b8e0c0e98

HACKING.rst

@@ -1,5 +1,5 @@
 Tacker Style Commandments
-================================
+=========================
 
 - Step 1: Read the OpenStack Style Commandments
   https://docs.openstack.org/hacking/latest
@@ -8,19 +8,19 @@ Tacker Style Commandments
 
 Running Tests
 -------------
-The testing system is based on a combination of tox and testr. The canonical
-approach to running tests is to simply run the command `tox`. This will
+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
-`testr run --parallel`, but is set up such that you can supply any additional
-testr arguments that are needed to tox. For example, you can run:
-`tox -- --analyze-isolation` to cause tox to tell testr to add
+``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 testr
-command directly. Running `testr run` will run the entire test suite. `testr
-run --parallel` will run it in parallel (this is the default incantation tox
-uses.) More information about testr can be found at:
-https://wiki.openstack.org/wiki/Testr
+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/