test: cover edge case for lockunspent rpc
This PR tests the scenario where a negative number is passed as the vout value during the lockunspent
RPC call. The rationale behind this test is to validate the input parameters for the lockunspent
function and ensure that the system behaves correctly when given invalid data. By confirming that the error message "Invalid parameter: vout cannot be negative" is returned, we can prevent unexpected behavior or vulnerabilities in the application.