Did a little test myself, only to debunk or confirm the charging AI.

Had a Legionary First Cohort against a unit of archers from Pontus. They did indeed close up dramatically, but when inside the pilumrange (and just outside the chargethreshold) they began to shoot.

It appears that Testudo only grants the shieldbonus to the exposed top and sides, not any other more fictive bonus, like Open formation grants a point of armour. But then again I mostly saw the arrows hit the front or the men on the sides.