Multiemployer ADP Testing

I have a multiemployer 401(k) plan that is comprised of 20 employers, with all participants collectively bargained (i.e., there are no non-union employees in the plan), and all part of the same CBA.  When performing the ADP testing for this plan, would I test each employer on its own or am I required to test the entire population as one employer?  The regs indicate that I must test the entire plan as one employer, but I could have sworn that testing for multiemployer plans was always done on an employer by employer basis.  (Meaning for this plan, if 19 of the 20 employers had no HCEs I could ignore those groups and just test the 1 employer that has an HCE).

Perhaps what I always thought was the case has simply always been wrong.  Any help is appreciated!

I would follow the reg (1.401(k)-1(b)(4)(v)(C) if it applies.