from Martincath: "All the tests which meet US standards for flying have detailed records of time/date and ID of person tested, whether it's an online Proctored or in-lab test, so there are records to prove you tested +ve and then left the country - once all the bureaucracy works through, expect to be banned from entering Canada ever again. Possibly fined too, just to add icing to the cake."
You don't need a COVID test to board a US flight from within the USA, so there is no "record" to connect a test to, and you woluldn't have a proctored test anyway.. I'm not advocating this, but just pointing out the inconsistencies of the governmental agencies trying to manage the situations. They are not in sync with plenty of loopholes.