Verse 1 sets the test, suggesting God wants to know something. (Notice the intent of God to "know" in 18:21, which also leads to a crisis.) It is not a game with God. God genuinely does not know. And that is settled in verse 12, "Now I know." There is real development in the plot. The flow of the narrative accomplishes something in the awareness of God. He did not know. Now he knows. The narrative will not be understood if it is taken as a flat event of "testing." It can only be seen as a genuine movement in history between Yahweh and Abraham.I think Brueggemann is right.
If we discount a genuine not knowing on the part of God in this story, we lose the significance of the narrative. God wants to know that Abraham will trust him to keep his promise to make of him a great nation even though the one through whom the promise will come (Isaac) is taken away. Abraham must trust God, but God must trust Abraham as well. Throughout the Abraham and Sarah narrative, the first Matriarch and Patriarch continually attempt to take control of the promise themselves (e.g. Hagar and Ishmael). God genuinely is not sure he can trust Abraham to leave it to God to fulfill the promise.
What makes this reading so difficult is when the text confronts our preconceived notions of omniscience and sovereignty. We try to squeeze the text to fit our doctrine of God. When we do that, we fail to let the text speak on its own terms and we perform hermeneutical gymnastics in our reading of Scripture. In addition, it makes God's relationship and interaction with his people appear to be nothing more than "smoke and mirrors" where God calls and leads and speaks even though he already knows the outcome. Is it possible to imagine God going through this entire scenario with Abraham and Issac, putting Abraham through this test with the rationale that God wanted to see if he could trust Abraham, even though God already knew that he could?
None of this explains what God knows about the future, whether he limits his knowledge of the future in order to be in genuine relationship with his creation, and what exactly God limits and what he truly knows of what is to come in our time and space which God is beyond. But in order to maintain the integrity of this biblical text from Genesis, we must take it at face value, and the text says that God does not know the outcome of his command to Abraham to sacrifice his son Isaac.
Who am I to say otherwise?