Add test documentation for docs tox target

Enable sphinx to generate documentation from docstrings by running
'tox -e docs'.

Change-Id: I5996e5f07493f69f14172b4bb0535852e89d5456
diff --git a/doc/source/test_descriptions.rst b/doc/source/test_descriptions.rst
new file mode 100644
index 0000000..240e448
--- /dev/null
+++ b/doc/source/test_descriptions.rst
@@ -0,0 +1,7 @@
+Description of Tests
+====================
+
+.. toctree::
+   :maxdepth: 2
+
+   tests/modules