Fix unreproducible tests

> The tests are CRAZY! They are even not reproducible. They pretty much ping a live server `electrum.blockstream.info:50001`. So I added the tests for the fees to make sure that the fee is `> 0`.

Maybe we should switch to use a local regtest electrs that we pre-populate with a certain amount of transactions and blocks to make them ~reproducible? Or, we could consider building a mock Electrum server, which shouldn't be hard to do? Especially given the confusion around fee rate conversion it would be good to test it properly in CI?

_Originally posted by @tnull in https://github.com/bitcoindevkit/rust-electrum-client/issues/136#issuecomment-2275080407_