Update test when 400 response is returned properly from GET request

The second part of this [test](https://github.com/stac-utils/stac-fastapi-elasticsearch-opensearch/blob/97dfde2591bca125ec351107082712ed86c899f6/stac_fastapi/tests/resources/test_item.py#L815-L834) has been commented out. Once there is a fix in stac-fastapi we should update the test. 
