I know its ahistorical. I haven't modified the single player export_descr_unit.txt as I feel its not worth doing. I did it for Custom Battles to experiment. If you want to try it I've attached it.
EB > mp game edu backup > export_descr_unit.txt
I had a few conditions before adding testudo as a formation. 1) They must not be spearmen, as it would be impractical to hold a spear and form testudo (unless the ranks behind the front dropped their spears on the ground). 2) They must be armed with a sword, axe, or club. 3) They must have a shield of certain size and shape. 4) No hoplon shield carriers can get testudo, when adding testudo to hoplon carrying units (pedites extraordinarii) the center ranks face their shields behind them instead of forward leaving large holes for missiles to get in. 5) No skirmisher units.
At first I wanted only trained and disciplined units only to get testudo but that limited my choices and left out a lot of good potential candidates.
Here is the list of units who got testudo:
Machimoi
Ethiopian Swordsmen
Dorkim Aloopim
Arjos
Bataroas
Botroas
Neitos
Golberi Curoas
Enoci Curoas
Gaesatae
Milnaht
Rycalawre
Solduros
Pictones Neitos
Teskatos
Kluddacorii
Teceitos
Cordinau Orca
Boii Swordsmen
Taxeis Triballoi
Komatai Epilektoi
Komatai Kostobokon
Galathraikes
Regional Galathraikes
Mardig Sooseramartik
Kartvelebi Dashna-Mebrdzolebi
Parthohellenikoi Thureophoroi
Pontic Thorakitai
Galatikoi Kleruchoi
Galatikoi Tindanotae
Herunautoz
Slanganz
Kirslininkas
Karotovas
Cheruscii Swordsmen
Wargoz
Agrianikoi Pelekephoroi
Thorakitai Argyraspidai
Loricati Scutari
Dunaminaca
Mercenary Dunaminaca
Ilergeta
Hastati (Camillan and Polybian)
Principes (Polybian)
Bruttian Infantry
Previews:
Boii Cingetos
Dunaminica
Enoci Curoas
Gaesatae
Golberi Curoas
Kluddacorii
Milnaht
Pictone Neitos
Solduros
Mardig Sooseramartik
Kartveli Dashnit Meombrebi
Gaesatae
Loricati Scutari
Pezoi Brettioi
Galathraikes
Taxeis Triballoi
Parthohellenikoi Thureophoroi
Galatikoi Tindanotae
Galathraikes
Pontikoi Thorakitai
Hastati (Camillan)
Hastati (Polybian)
Worgōzez
Xerunoudōzez
Happy Besieging!... or happy standing!