test: Share syncer test setup between MerkleDB and Firewood

[alarso16]

It turns out that the key length restriction put in place here was only necessary due to a bug in Firewood, which is now fixed. I think we can reduce the duplicate code, but it would touch quite a bit of merkledb code too. What if after we merge this PR, I refactor all the testing to work for both databases? I think most of the benefit will come once we want to start testing change proofs as well. I worry the scope of refactoring all these tests will creep into implementation

Originally posted by @alarso16 in #4361

[github-actions]

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