Add docs on running tests

docs/contributing/code-review.rst

@@ -46,4 +46,6 @@ feels exhaustively unit-testable, write unit tests too. When in doubt,
 add more tests.
 
 If you are unsure what kind of tests to add for your pull request, other
-contributors to the repo will be happy to help guide you!
+contributors to the repo will be happy to help guide you!
+
+See :ref:`contributing/tests` for guidelines on writing tests.

docs/contributing/tests.rst

@@ -0,0 +1,59 @@
+.. _contributing/tests:
+
+============
+Testing TLJH
+============
+
+Unit and integration tests are a core part of TLJH, as important as
+the code & documentation. They help validate that the code works as
+we think it does, and continues to do so when changes occur. They
+also help communicate in precise terms what we expect our code
+to do. 
+
+Integration tests
+=================
+
+TLJH is a *distribution* where the primary value is the many 
+opinionated choices we have made on components to use and how
+they fit together. Integration tests are perfect for testing
+that the various components fit together and work as they should.
+So we write a lot of integration tests, and put in more effort
+towards them than unit tests. 
+
+All integration tests are run on `CircleCI <https://circleci.com>`_
+for each PR and merge, making sure we don't have broken tests
+for too long.
+
+The integration tests are in the ``integration-tests`` directory
+in the git repository. ``py.test`` is used to write the integration
+tests. Each file should contain tests that can be run in any order
+against the same installation of TLJH.
+
+Running integration tests locally
+---------------------------------
+
+You need ``docker`` installed and callable by the user running
+the integration tests without needing sudo. 
+
+You can then run the tests with:
+
+.. code-block:: bash
+
+   .circleci/integration-test run-test <name-of-run> <test-file-names>
+
+
+``<name-of-run>`` is an identifier for the tests - you can choose
+anything you want. ``<test-file-names>>`` is list of test files
+(under ``integration-tests``) that should be run in one go.
+
+For example, to run all the basic tests, you would write:
+
+.. code-block:: bash
+
+   .circleci/integration-test.py run-test basic-tests \
+                                 test_hub.py \
+                                 test_install.py \
+                                 test_extensions.py
+
+This will run the tests in the three files against the same installation
+of TLJH and report errors.