I think everyone's idea of what constitutes an emissions test is different. Using the google machine, it appears 34 States require vehicle emissions testing.
Of the 34, I have no idea if any still do a tailpipe emissions test. I live in a PZEV State and we don't do them. Never did as far as I know.
It would be interesting to know how long a OBDII compliant vehicle could pass a tailpipe emissions test, given it's running as designed, with no MIL on, no codes, etc.
I can tell you what is needed in Belgium: For a gas enfine there's a sniffer placed up the tailpipe and it measures O2, Co, CO2, HC and displays lambda. CO has a max limit (0.5% i would think) and Lambda has to be between 0.97 an 1.03. Some cars get tested at idle others at 2500 rpm, depends on the year of manufacture.
If the car smokes excessively they can fail it for that even if you pass the sniffer test.