Am I allowed to testify that I did not hear screaming on the stairwell of the building, knowing that I was not present on that day, in order to acquit my friend who is accused of harassment and whose innocence I trust?

As long as the questioner was not present, he cannot testify to something he did not witness, because the judge would understand from his testimony of not hearing the plaintiff's screams that he was present, which contradicts the reality of the situation. Ibn Qudamah stated in "Al-Mughni": "Testimony is not permissible except for what one knows, based on the Almighty's saying: {except for those who bear witness to the truth, and they know} [Quran 43:86], and His saying: {And do not pursue that of which you have no knowledge. Indeed, the hearing, the sight, and the heart - about all those [one] will be questioned} [Quran 17:36]." What the questioner can testify to is only the good character of his neighbor and that there were previous disputes between his neighbor and the plaintiff.