Add legacy wallet tests

[realeinherjar]

This discussion came up on bitcoindevkit/bdk#1115.

Currently we are only testing segwit wallets:
https://github.com/bitcoindevkit/bdk/blob/f95506ba6a4a92e977ad228114683164c4ca1222/crates/bdk/tests/common.rs#L107-L156

Do we want to support legacy wallets?
If yes, we need legacy wallet tests.

[ValuedMammal]

A couple ideas for this: we should change the assert_feerate macro to just a plain vanilla function - right now it's difficult to read and not very approachable. It should take in an optional descriptor string to calculate the satisfaction weight instead of assume the same P2WPKH_FAKE_WITNESS_SIZE for every psbt. We can add some legacy descriptors to wallet/tests/common.rs and then come up with a similar assert_feerate_legacy that contains logic specific to pre-segwit transactions.