@Ken-And-Denise0 it's not unreasonable to request, as long as you accept that the guests will be fully refunded if one of them gets a positive test result, or if you turn them away for declining to provide a negative test. Also, speaking purely in terms of hospitality rather than personal safety, adding new requirements to a pre-existing booking may be off-putting to some, especially those guests who dwell in the alternate reality where the pandemic is over.
If you offer a self-contained space, your best defense remains social distancing and buffer time before changeovers. A test result is only a snapshot of one moment before the stay and provides no useful indication of your risk level as the stay progresses, so you can't really put much into it.