test(wallet): update to use new SignerWrapper util

- updates the existing tests that uses the `Wallet::sign` API, to gethe
  signer implementation from test utilities, either:
`get_wallet_signer_single` or `get_wallet_signer`.
- there are a few tests which needs further refactoring, as they rely on
  the use of `Wallet::finalize_psbt`.

Author: oleonardolima