"If a prophet or a dreamer of dreams arises among you and gives you a sign or a wonder, 2and the sign or the wonder comes true, concerning which he spoke to you, saying, ‘Let us go after other gods (whom you have not known) and let us serve them,’ 3you shall not listen to the words of that prophet or that dreamer of dreams; for the Lord your God is testing you to find out if you love the Lord your God with all your heart and with all your soul," (Deut. 13:1-3).
This is an example of a testing situation. God routinely tests people--not so that God will learn what they will actually do but because God knows all things (1 John 3:20). Instead, it is for the strengthening and purification of the faith of God's people, so they might learn and grow through the test. Furthermore, God speaks to them in discovery terms so that they might hear and know that God "now will use them" according to His purpose. Testing verses are also related to God's anthropomorphic expression where God speaks in human terms in human ways as He reveals Himself to and through people.
If we were to look at this from the perspective of open theism, we have a problem. Open theism teaches that God knows all things in the present--exhaustively. Deut. 13:1-3 tells how God can test people to find out if they love the Lord with all their heart. But if God knows all things in the present, then didn't He know if the people loved Him or not? Of course He did. So, this is a problem for their position.
Instead, God is testing the people, so they will know what is in their own hearts.