What does it mean that the righteousness of God without the law is witnessed by the Law and the Prophets?
It means God's righteousness apart from the law has been made manifest and put in the spotlight, with the Old Testament Law and Prophets bearing witness to it. This righteousness is not attained through law-keeping; Scripture foreshadowed and testified to it before Paul's later revelation to the Gentiles.
Les explains that "the righteousness of God without the law" is manifested, or put in the spotlight, and is witnessed by the law and the prophets. He traces the progressive revelation through Genesis, the flood, the Tower of Babel, Abraham, the covenant promises, Isaac, Jacob, Israel coming out of Egypt, the law under Moses, and then the prophets pleading with Israel. Prophecy was fulfilled to a great degree when Christ came the first time, yet Israel under Peter and the eleven continued to reject the offers. So God saved another little Jew and sent him to the Gentiles, showing the difference between law and grace.