Testudo was not the standard fighting formation of the Roman legions. "Testudo" refers to a very specific form of formation wherein Roman legionaries would form a compact block and use their shields to cover the top and sometimes sides of their formation as well as the front. Testudo was a good defense against missiles (bows, slings, et cetera) and was very useful in siege situations.

Then the shield-bearers wheeled about, enclosing the lighter armed troops within their ranks, while they themselves dropped on one knee and held their shields out before them. The second rank held their shields out over the heads of the first, and the next rank likewise. The resulting appearance is very like that of a roof,41 affords a striking spectacle, and is the most effective of protections against arrows, which glide off from it

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Antony*.html

Roman soldiers would not actually remain in testudo formation whilst fighting against the enemy in melee combat. It's simply too unwieldy of a formation. Why would you fight the enemy in melee combat whilst holding shields above your heads? The testudo is a purely defensive formation.

Essentially, the testudo was a specific sort of formation to deal with a specific sort of threat. It was not the primary fighting formation of the Roman soldiers. In comparison, the phalanx was the primary fighting formation for Hellenistic soldiers. The phalanx is quite obviously intended for melee combat, whilst the testudo is not.

In a hypothetical battle between the Romans and a phalanx-using Hellenistic army (or one of the real battles between Rome and the Hellenistic successor states, like Pydna or Magnesia) , Roman soldiers would not have been arranged in the testudo formation in order to fight the enemy phalanx. There were multiple different fighting formations used by pre-Marian and post-Marian Roman infantry - most of which did not involve holding their shields above their heads.