That's not quite the point.There have been more than enough cases that -ve became +ve after few days or even longer!!!!!
NO test kit is 100% accurate and the virus has long incubation period inside the host.
Even if the test were 100% accurate, it is only applicable to that moment in time when it was taken. You might have been exposed prior to that but the viral load has not yet developed (your "incubation time"), or you might get exposed after the test.