WIP: fix withdraw reduce balance and add initial tests

[ltfschoen]

Why is this error called Result::unwrap()on anErr value: GenericErr { msg: "Invalid hash format" } occuring when i use Rust Nightly and run cd ./packages/secret-contracts/nunya-contract && cargo test

The error is occuring on this Line 829 of code https://github.com/svub/nunya/pull/37/files#diff-d6a7ac3c294e496b72f7d1f5bdecff8f47d478f755c2ae3c04e695690020e950R829, and i believe the cause is because i haven't used the right format for the input_hash or signature property values of the PrivContractHandleMsg struct on Line 824-825. But even in this example https://github.com/SecretSaturn/SecretPath/blob/main/TNLS-Samples/Storage/src/contract.rs#L399 they also use the same format for those values.

I tried changing the hash value to Region::from_slice("message_hash".as_bytes()) by using use cosmwasm_vm::memory::{Region};, but it wouldn't let me import that since use cosmwasm_vm::memory is a "private module".

733 |     use cosmwasm_vm::memory::{Region};
    |                      ^^^^^^ private module

Error

---- contract::tests::withdraw_reduces_withdrawal_amount_from_payment_ref_storage_balances stdout ----
thread 'contract::tests::withdraw_reduces_withdrawal_amount_from_payment_ref_storage_balances' panicked at src/contract.rs:829:75:
called `Result::unwrap()` on an `Err` value: GenericErr { msg: "Invalid hash format" }
stack backtrace:
   0: rust_begin_unwind
             at /rustc/798fb83f7d24e31b16acca113496f39ff168c143/library/std/src/panicking.rs:665:5
   1: core::panicking::panic_fmt
             at /rustc/798fb83f7d24e31b16acca113496f39ff168c143/library/core/src/panicking.rs:74:14
   2: core::result::unwrap_failed
             at /rustc/798fb83f7d24e31b16acca113496f39ff168c143/library/core/src/result.rs:1698:5
   3: core::result::Result<T,E>::unwrap
             at /Users/luke/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/result.rs:1103:23
   4: nunya::contract::tests::withdraw_reduces_withdrawal_amount_from_payment_ref_storage_balances
             at ./src/contract.rs:829:13
   5: nunya::contract::tests::withdraw_reduces_withdrawal_amount_from_payment_ref_storage_balances::{{closure}}
             at ./src/contract.rs:805:78
   6: core::ops::function::FnOnce::call_once
             at /Users/luke/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/src/rust/library/core/src/ops/function.rs:250:5
   7: core::ops::function::FnOnce::call_once
             at /rustc/798fb83f7d24e31b16acca113496f39ff168c143/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

Note that I've also been using this codebase for reference https://github.com/CosmWasm/cosmwasm/blob/main/contracts/hackatom/src/contract.rs