[#1483] add extension writing tip about avoiding plugin importing

doc/extensions/testing-extensions.rst

@@ -61,6 +61,12 @@ Some notes on how these tests work:
 
 * The Pylons book also has a `chapter on testing <http://pylonsbook.com/en/1.0/testing.html>`_.
 
+* Avoid importing the plugin modules directly into your test modules 
+  (e.g from example_iauthfunctions import plugin_v5_custom_config_setting).
+  This causes the plugin to be registered and loaded before the entire test run,
+  so the plugin will be loaded for all tests. This can cause conflicts and 
+  test failures.
+
 .. todo::
 
    Link to CKAN guidelines for *how* to write tests, once those guidelines have