MollomDataTestCase does not properly override testing user

I attempted to fix this a couple of times already, finally figured out how to fix it properly.

MollomDataTestCase tests the API function mollom_form_get_values() directly, without performing requests to the internal testing site setup by SimpleTest.

Because of that, actions like $this->drupalLogin() do not have the desired effect -- the given user is authenticated in the testing site, but not the "parent" site that is running the test.