This PR adds missing coverage for the bumpfee RPC, for the case that a wallet transaction is passed with an input that is already spent:
https://github.com/bitcoin/bitcoin/blob/0b02ce914e8594e8938e527c91c07f57def4e943/src/wallet/feebumper.cpp#L182-L186
This is achieved by simply creating a transaction with a wallet and then mining it (I'm not aware of any other scenario how this could be achieved). Additionally, two RPC throw checks are changed in the test to be more specific: https://github.com/bitcoin/bitcoin/blob/0b02ce914e8594e8938e527c91c07f57def4e943/src/wallet/feebumper.cpp#L42-L45 https://github.com/bitcoin/bitcoin/blob/0b02ce914e8594e8938e527c91c07f57def4e943/src/wallet/feebumper.cpp#L47-L50