At the moment it is not possible to delete a candidate card (test or regular) from a job opportunity.
We are working on adding an option to delete a candidate from an opportunity.
In the meantime, you can try this workaround:
Create an custom offline stage and name it Archived or whatever you like.
Then the expired cards you should be able to drag to that stage. No need to reject expired candidate cards.
If you do want to reject someone who completed the assessment and want them out of view from that stage, you can drag the card to the dedicated offline stage, and then reject them from that stage.
Please note: If you reject them while in the original stage, the card will appear faded and you won't be able to move it to another stage.