'Regression test for #18051'

def test_flatten_fieldsets(self):

    fieldsets = ((None, {'fields': ('url', 'title', ('content', 'sites'))}),)
    self.assertEqual(flatten_fieldsets(fieldsets), ['url', 'title', 'content', 'sites'])
    fieldsets = ((None, {'fields': ('url', 'title', ['content', 'sites'])}),)
    self.assertEqual(flatten_fieldsets(fieldsets), ['url', 'title', 'content', 'sites'])
