Yes, if you delete the session or the session expires, you will regain the test credits for any tests not completed.
Example: 3 tests in the session and none have been completed. Deleting (or expired) the test session will return the test credits to your inventory balance.
Example: 2 tests in the session, one of which was completed and one incomplete. Deleting that test session will return only one (1) test credit back for the incompleted test, as the other test was completed.