bark: Prevent estimation API from erroring due to lack of funds

I made some tweaks to the estimation API. Methods which relied upon `select_vtxos_to_cover` would error if the wallet doesn't have enough funds. This isn't particularly useful for estimations which wallets may which to display before a user tries to perform an operation. I changes the behaviour for these methods to always return the highest fee possible for the given amount which means users who add more funds to their wallet will either pay the correct fee or less than they expect which I think is a good idea.

Additionally, I added estimation to each fee integration test and validate the end result against the estimation to ensure the two always stay in sync.

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