Buy the cheapest ticket with no travel insurance as you always would.

If you happen to draw the unlucky straw on the test, you will have to recover and test positive, After that you buy a new ticket and only pay the price difference which will likely be almost $0.

If you really do nee to re-book, it can be done online if you know what you are doing (you need to apply the e-cert). Or you can call in and explain. Check back here in the unlikely case this happens.