MySQL - A Count with INNER JOINS - that multiplies the result by 8
Perhaps you want something like this?

SELECT tu.*
FROM tests t
JOIN testusers tu ON (tu.TestNumber = t.TestNumber)
WHERE t.TestComplete = 0 AND  -- below is your tricky bit
      (ResignedTimestamp='1970-01-01 00:00:01' OR
       NOT EXISTS (SELECT 1 FROM testusers WHERE TestNumber = tu.TestNumber AND UserSlot = tu.UserSlot AND UserId &lt;&gt; tu.UserId))
ORDER BY tu.UserId