'Registering a new serializer populates the full registry. Refs #14823'

def test_register(self):

    serializers.register_serializer('json3', 'django.core.serializers.json')
    public_formats = serializers.get_public_serializer_formats()
    self.assertTrue(('json3' in public_formats))
    self.assertTrue(('json2' in public_formats))
    self.assertTrue(('xml' in public_formats))
