A 'test bench' is nothing more than a motherboard tray on a stand with or without attachments for mounting an aio, water cooling etc. Everything else like the psu just sits on the bench/table etc. There's no place for the power switch unless you add one etc. As said, it's just a bad idea all around unless it serves a realistic purpose.
A case actually does serve a purpose by directing airflow, thermal currents etc which can benefit motherboard components including ram, VRM's, Northbridge/pch etc. You'll not get that on a testbench unless there's some active airflow across the motherboard, part of the reason testers use a large aio that's mounted on the side of the bench where intake fans would be in a case.