test(proof): add failing case

[tbruyelle]

Following convo on Signal, I create this draft PR to demonstrate the bug and report all the informations.

Disclaimer

I didn't test all the different scenarios so there might be other cases of proof generation that are incorrect. The case I present here is the one I had a chance to reproduce, but I found an other scenario where the proof was also incorrect but differently than the case here.

Context

While I was asserting the behavior of tm2 proofs (for the IBC connection with A1), I found an issue with absence proofs.

The test TestTreeGetWithProof() (modified in this PR) builds a merkle tree with keys 0x11, 0x32, 0x50, 0x72, 0x99, and so looks like this:

• Node label is KEY / HASH
• The first two leafs are numbered with 1 and 2 for the purpose of the demo.

After asserting a proof of the presence of the key 0x32, which is not relevant here, the test asserts a proof of the absence of key 0x1. The returned proof is correct and asserts that 0x1 is absent of the tree. In term of leaves, the proof contains only the leaf 1, because 0x1 < 0x11 (key of 1).

Now if that key is changed to 0x1100 like in this PR, the returned proof is exactly the same as with 0x1, but this is incorrect because this time 0x1100 > 0x11. For that proof to be correct, the proof's leaves should contain both 1 and 2 because 0x1100 is between 0x11 and 0x32.

Hence the verification algorithm returns an error, given that absence of second leaf: absence not proved by right leaf (need another leaf?)

Important

The same change applied to the same test in cosmos/iavl produces a valid proof.

[Gno2D2]

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[tbruyelle]

Fixed by #4656