[FIX] config: add a config dest to test-enable option

Commit 76c5389 removed the option dest for test-enable leading to a
traceback when trying to save the configuration.
This was due to the fact that the options dict was sorted while
containing a None key because of the missing dest.

odoo/tools/config.py

@@ -157,6 +157,7 @@ class configmanager(object):
         group.add_option("--test-file", dest="test_file", my_default=False,
                          help="Launch a python test file.")
         group.add_option("--test-enable", action="callback", callback=self._test_enable_callback,
+                         dest='test_enable',
                          help="Enable unit tests.")
         group.add_option("--test-tags", dest="test_tags",
                          help="Comma separated list of tags to filter which tests to excute. Enable unit tests if set.")