Update testing.md file.

How to run Python tests for stub servers.

docs/en_us/internal/testing.md

@@ -165,6 +165,13 @@ or to get html report:
 
 then browse reports/common/lib/xmodule/cover/index.html
 
+To run tests for stub servers, for example for
+[YouTube stub server](https://github.com/edx/edx-platform/blob/master/common/djangoapps/terrain/stubs/tests/test_youtube_stub.py),
+you can do one of:
+
+    rake fasttest_cms[common/djangoapps/terrain/stubs/tests/test_youtube_stub.py]
+    python -m coverage run --rcfile=cms/.coveragerc `which ./manage.py` cms --settings test test --traceback common/djangoapps/terrain/stubs/tests/test_youtube_stub.py
+
 
 Very handy: if you uncomment the `pdb=1` line in `setup.cfg`, it will drop you into pdb on error.  This lets you go up and down the stack and see what the values of the variables are.  Check out [the pdb documentation](http://docs.python.org/library/pdb.html)