We have a related modern question:
But if we move back into antiquity, are there accounts of the testudo formation withstanding ballistae bolts? Could it?
The Roman Scutum (I assume this is the shield you are referencing to) has been reported to not stand up to the bows in use by the Parthian's in Plutarch's Life of Cassius, in the following line:
6 Thus many died, and the survivors also were incapacitated for fighting. And when Publius urged them to charge the enemy's mail-clad horsemen, they showed him that their hands were riveted to their shields and their feet nailed through and through to the ground, so that they were helpless either for flight or for self-defence.
It is hard to rationalize that the much larger Ballista would have a harder time of piercing a Testudo formation than that of the much smaller Parthian bows. As there were no sources I could find of Ballistas being used against the Roman Scutum.
Here are the facts on ballista's strength and weights data available from Wikipedia:
To compare these values to normal bows, here are these two measurements for a typical 50 pounds bow:
These two facts makes simply impossible to imagine anybody to stop these projectiles with a hand held shield without getting damaged. The force of these projectiles were lot more than the roman shield could stop, and based on facts the projectiles would go through the testudo, piercing through the first persons shield and knocking the standing people behind by the first person. Their only hope could be only if it does hit only a shield and then the ground, but in testudo it wasn't very likely since the closed formation.
Source: Plutarch's Life of Cassius