This is surprising behavior, and will generate non-standard transactions with 2 OPRETURN outputs if we try to use the `target_size` parameter in `MiniWallet.create_self_transfer` if the wallet is not in RAW_P2PK mode. The only reason it currently works in RAW_P2PK mode is that the tx is alreadly larger than 100 bytes and `pad_tx(tx, 100)` is a no-op.
Adjust the test that use `MiniWallet.sign_tx` and previously relied on that tx padding implicitely. Note that `pad_tx` already take care of updating the tx hash, so no need to call `tx.rehash`.