Perhaps a vocal music recorded in a large quiet church where you can hear the music decay to the background noise level, and recorded at -40 dBFS would be a good test.
The point is not that I can conceive of a test that might show the audible difference between the A/D section of a $2000 and $200 recorder. Rather, I think it is more significant that an extraordinary test is required. There are many good inexpensive (and expensive) recorders out there. Maybe there are more important aspects of recording to focus on. I like gear as much as the next guy, but so often we are too focused on the "noise". The problem with my recording are that I placed the mics too far away, or maybe the flute player should be turned 45 degrees, or the HVAC system is way too loud, or I'm picking up RF, or .... Never have I thought "this recording would be perfect if only my A/D was better".