Firstly, you speak as though Jesus Christ is only man! He IS The God-Man, in ONE Person. This Person cannot be divided into two, which is a grave heresy. Jesus' testing by the devil was done in the hope that Jesus might fail. How many times we read of people going out to war against a country that is far stronger and most certainly they would be defeated? the OT is full of such accounts. However, these weaker nations still went out to battle, in the HOPE that they MIGHT win, even though this was IMPOSSIBLE from even before they went to battle! The devil already fully knows, as he has seen this in the Bible, that he WILL be defeated by the Lord Jesus Christ as it says in the Book of Revelation, and YET, he is still going to fight! Why, you may ask, if he already knows that he will 100% be defeated! Likewise with the testings of Jesus, the devil knew that Jesus Christ is Almighty God, Who remained God and became the God-Man, and that He IS Invincible in His Person. But, since the devil is THE deceiver, he himself got deceived!