Richard,
At first, I thought insurance companies offering temporary cover will not quote you for one day's worth of insurance for your instructor test because that's not this deviates from the norm. However I'm happy to tell you that when I researched for specialist companies for driving instructor insurance, I managed to come across one which will insure people who are going for either the DSA Part 2 or the DSA Part 3 Check Test. The company in question is Waveney Group Schemes Ltd and you can purchase your ADI check test insurance through this link.
Let me know how that goes for you.
