Opt-outs (DNS) like that simply need to be tested. As are DNF's. Show up to the finish line to check whether you're being tested. Some have gotten suspension by DNF'ing and going home.
I preferred the type of testing where we were already racing and during the race they showed up and the speaker mentioned the random starting number to be tested, apart from the podium.
Sometimes those random tests managed to get someone who had a shady name. Good to have those tested, for many reasons, even if it's a negative result.