I had this concern when we were in Canada over the holidays, especially when I came down with an actual cold (or cold-induced rhinitis).
We bought the BinaxNOW tests that come with the zoom call. There were only two of us, so we brought all six.
If I tested positive, I was planning to use up all of those before going for a local test. I had four extra tests, so I figured that would get me an extra 3-4 days (testing once a day). Looking back, the extra tests were a nice cushion to have.
Let's just say the "testing protocols" are not very strict with these tests. Unless you had a very strong/dark second line, I'm not sure the proctors would be able to see it.
Also, as a side note, no one even checked our Covid test results before we boarded the plane to come home - not the US agent at the Calgary airport and no one from the airlines.