I did figure out a workaround for this, but it is still unwieldy, and we really need to get this feature request filled.
This will work if you have one quiz ("the opening quiz") to open one more quiz ("the second quiz"). It may work for longer chains, but I haven't tested that.
Go to "Grades" and assign a 100% grade to the Test Student for "the opening quiz". In Modules, click on "the second quiz", and you will see a new date box assigned to the Test Student. Set the dates in this box as desired. Save and click Continue in the warning box.
I know, I know...it's not ideal, but it's better than having to remember to go back in and set a date after a real student has taken "the opening quiz".