Fix custom invalid error message (also fixes 1.6 compatibility).

The custom "invalid" error message for `ArrayFormField` was never being used, because it is supposed to be specified in a class attribute called `default_error_messages`, not `error_messages`. `self.error_messages` is set in `Field.__init__`.

This bug was sort of hidden in Django 1.5 and previous; the custom message was not used, but there was a default "invalid" message, and so no error occurred. In Django 1.6 `Field` no longer has an "invalid" error message by default, and so things blow up.

This pull request fixes it for all Django versions (and fixes a typo in the name of a testing form class, and adds a `TEST_RUNNER` setting so that the tests continue to work on Django 1.6 as they do now).