Add test_suite variable for test directory.

setup.py

@@ -27,5 +27,7 @@ topydo is a todo list application using the todo.txt format. It is heavily inspi
 * Maintain dependencies between todo items;
 * Allow todos to recur;
 * Some conveniences when adding new items (e.g. adding creation date and use relative dates)
-"""
+""",
+
+    test_suite = "test",
 )