BS.1534 Configs for Binaural Listening Tests: preprocessing.loudness_fmt should be set to "BINAURAL"

In BS.1534 binaural listening tests, when preprocessing.loudness_fmt and preprocessing.fmt are not specified, the listening items will be rendered to 7.1+4 by default (in ivas_processing_scripts/audiotools/wrappers/bs1770.py::get_loudness()) before loudness is measured. However, in binaural listening tests, such as, BS1534-4a.yml, preprocessing.loudness_fmt should be explictly set to "BINAURAL", so that the loudness measurement is performed on the binaural signal.