In the spirit of "do the simplest possible thing that works..." I think I see a very simple thing that will work.
The simple additional rule for the testnet:
If current_block_timestamp - previous_block_timestamp > 20 minutes:
block difficulty = minimum difficulty
And that's it.
If mining is proceeding normally and most blocks are produced in less than 20 minutes, then the rules are exactly the same as the main network.
But if somebody has driven difficulty way up, then the new rule acts as a safety valve, ensuring that new blocks are created at least once every 20-something minutes or so. After a month of "safety valve blocks" the difficulty would be calculated as normal, and would get cut in (approximately) half.
This does make the testnet block chain more susceptible to forks -- somebody with a bunch of hashing power can pretty easily invalidate a long chain of 20-minute, min-difficulty blocks if they want to. And there is likely to be a flurry min-difficulty blocks produced/announced every 20 minutes. But that could be considered a feature (test your block-chain-reorganization code!).