What should I do, and is there repentance for me, knowing that I testified to a loan contract at the bank without my knowledge of the ruling on false testimony, and without my knowledge of my friend's intention?

If ignorance of the impermissibility of bearing witness to a usurious contract is an excuse, then ignorance that it is one of the major sins for which the doer is cursed does not remove the effect of the sin. What was obligatory upon you was to forbid your friend from borrowing with usury, and to refrain from bearing witness to this forbidden contract. Now, the door of repentance is open. Repent to Allah with sincere and truthful repentance, resolve not to repeat the sin, and regret what you have done. If your repentance is truthful, Allah will accept it and forgive you. And you should advise your friend and explain his error to him.