Include BDK integration tests in rust-payjoin

[spacebear21]

Currently rust-payjoin uses bitcoind for integration tests, and payjoin-ffi uses BDK for its integration tests.

Instead, we can ensure compatibility with BDK by including it in tests at the rust-payjoin library level, and use simple bitcoind tests to test each binding layer.

See also the conversation in https://github.com/LtbLightning/payjoin-ffi/issues/11

[shinghim]

Does this mean duplicate the existing rust-payjoin tests that use bitcoind but for BDK, and vice versa for the binding layer tests?

[spacebear21]

We just need to bring in the BDK test from payjoin-ffi into rust-payjoin. After https://github.com/payjoin/rust-payjoin/issues/790 we will no longer really need to test the payjoin-ffi wrappers directly, instead we can just have integration tests in each generated language.

I think it's beneficial to test BDK integration in the core rust-payjoin library since it's so commonly used in Rust projects, but for language bindings the tests are really just there to prove that the bindings work as intended and to provide example usage, so IMO it's fine to just use bitcoind for these.