Document use of --disable-migrations flag

docs/en_us/internal/testing.rst

@@ -167,6 +167,12 @@ To run these tests without ``collectstatic``, which is faster, append the follow
 
     paver test_system -s lms --fasttest
 
+For even more speed, use the ``--disable-migrations`` option to run tests without applying migrations and instead create tables directly from apps' models.
+
+::
+
+    paver test_system -s lms --disable-migrations
+
 To run cms python tests without ``collectstatic`` use this command.
 
 ::