The flights that have been carried out by KLM and LH can hardly be classified as test flights. They have merely taken a snapshot of the conditions and found that there was no hazard to those particular flights only. It is laughable for them to use those 'findings' (used loosely) and base their proposed operational program on that.
Even if airspace was opened again, should any dense ash clouds track towards Europe and with the best intentions of getting aircraft moving, the slot and flow implications would be horrendous in avoiding the worst of the airborne deposits.
I am sure the insurance companies will have something to say about this too!
If KLM/LH manage to pull the wool over the eyes of the media, public and worst of all the industry, it will be rather worrying day!