The test proofs are generated in JS using the Open Zeppelin example script verbatim.
The OZ scripts can be found in the README at:
https://github.com/OpenZeppelin/merkle-tree/tree/98ff55dc7b6004a6e57ca0cb256cc0d08ecb6096
This linked commit is HEAD as of time of writing.
The proofs build deterministically and so are included in the rain-merkle-prelude nix task, and are committed to the repo.
The outputs are copied to Solidity constants in test/proof/LibTestProof.sol.