@Jennifer35 You can require whatever you need to feel comfortable, however, if this requirement was not in place at the time of the booking, it might cause problems as it was not a condition that existed when the reservation was made. If there was subsequent communication allowing the guest time to comply and the guest refused to do so, then you might be able to cancel with extenuating circumstances. However, being able to be tested, especially without symptoms, might problematic for any guest. Also the negative test is only a results for the time the test was taken and the guest could be infected subsequent to that.
If this is a long term guest, perhaps you can express concern and seek feedback from the guest as to how to address the issue. The guest is coming into a home, that while it may have been properly cleaned, the guest has no idea about your status.
In my city, shared home listings are not allowed to accept guests. I actually agree as I would rather not add that factor into my life at this time.