bdk-swift Tests folder

[reez]

Describe the bug

We removed Tests folder in bitcoindevkit/bdk-swift@d2a2073 so we can remove .testTarget(name: "BitcoinDevKitTests") in our Package.swift in https://github.com/bitcoindevkit/bdk-swift

Do not need to change bdk-ffi/bdk-swift though.

Additional context

Historically https://github.com/bitcoindevkit/bdk-swift is consumed as a remote SPM dependency or worked on locally from https://github.com/bitcoindevkit/bdk-ffi/tree/master/bdk-swift

But if someone were to happen to do a fresh clone https://github.com/bitcoindevkit/bdk-swift and then build they would run into an error locally