Expose a _test_utils feature to expose integration test helpers

[DanGould]

Higher layer payjoin implementations like payjoin-ffi and payjoin-flutter and the bindings they produce would make use of a _test_utils feature like rust-lightning has. The _ prefix prevents it from being a listed feature in documentation by default.

This should expose:

• BitcoinD and wallet setup
• payjoin-directory setup and teardown, including ephemeral Redis containers
• ohttp-relay setup and teardown

Then, the higher level crates can create bindings to make these available in all languages we bind to for easy test duplication downstream.