What is the significance of Deuteronomy 13:1–3?

Moses tells the Israelites that God allowed false prophets to sometimes be correct because “the Lord your God is testing you, to know whether you indeed love the Lord your God with all your heart and soul.”

If God already knows such matters with certainty, Scripture’s inspired description as to why such testings take place (viz. for God “to know…”) cannot be correct. If we believe that Scripture cannot err, it seems we should conclude that God does not necessarily foreknow such matters after all.