Lexie's point above correlates with something that seems feasible with my STBXAH, too. When he's late or misses a test, and then offers what seems to be a plausible excuse, he's 'grooming' me so that when he really is drunk, I won't be able to call him on it. After all, if I didn't challenge him last time . . . (ugh).
I love Puzzled's idea about reversing the expectation to a bonus rather than an onus. If my STBXAH wants to prove he's sober, he, too, should be acing the tests.