Fix unreproducible tests

[storopoli]

          > 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 #136 (comment)

[oleonardolima]

I do like the idea of using a local regtest electrs, by using electrsd for example.

I also think it'll be pretty much straightforward if we choose to use our bdk_testenv crate instead of trying to rebuild everything from scratch again with electrsd.