More than you ever wanted to know about Abraham's trials

What might be one of the earliest recorded midrashim (by which I mean an interpretation of biblical text) appears in the book of Nehemiah. The Levites (or perhaps Ezra) are addressing the people, with something that seems to be both blessing and history lesson, when they say:

          You are the LORD God, who chose Abram and brought him out of Ur of the Chaldeans and named him Abraham. You found his heart faithful to you...

As the ancient interpreters read it, this sentence suggests Abraham was tested. How else, they reasoned, would God have discovered that his heart was faithful? This idea of a tested Abraham appears (according to James Kugel) in apocrypha including Ben Sira, Maccabees and Jubilees, before it was recorded for posterity in Bereishis Rabba (The other books are older, lost, and exist today only in translation.)

Now the puzzle: Does the verse in Nehemia refer to the tradition of Avraham's trials, or was that tradition later devised based on this verse? What I mean is: Did the speakers in Nehemiah 9 know about this tradition of trials, and have it in mind when they spoke the words, or did they intend something else entirely that was, nonetheless, understood by later interpreters to be a reference to Abraham's ordeals? Did they create the tradition, or did they know it?

I favor the idea that the tradition of Abraham's trials is older then Nehemiah, but I don't know how to go about proving it.

Additional fun fact to know and tell:  There's an old tradition that Abraham was tossed into a furnace by a king who objected to Abraham's monotheistic beliefs. The story of the furnace seems to have been devised as  a solution to an ambiguity in Issiah 29:22 where it says God redeemed Abraham using the word pada which can also mean "rescued." But from what was Abraham rescued? Answer: the furnace in Ur Casdim. [Update: Josh Waxman points out that the heh of the definite article in Gen 19:28 (haKivshan) is additional evidence of a significant furnace in Abraham's past.] Moreover, our verse in Nehmiah seems to support all of this. "Ur" is not just the name of Abraham's city. It is also a word for flame, or fire, thus Nehemiah 9:7 becomes:  You are the LORD God, who chose Abram and brought him out of the fire of the Chaldeans. 

This, I expect you will be tickled to learn, is exactly how the Vulgate translates Nehemiah 9:7 into Latin, suggesting Jerome took the story of Abraham escaping the furnace as pshat in the verse!

Still another fun fact to know and tell: We all take it for granted that Abraham was the very first monotheist, but this is nowhere directly represented in the text. Like the idea that Noah preached repentance for 120 years, Abraham's monotheism is an interpretation -- not necessarily an incorrect interpretation, but an interpretation, nonetheless.

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