Relocate balanceTransactionSpec to Cardano.Write.Tx.BalanceSpec. (#…

…4145)

## Issue

ADP-3171

## Description

This PR is the **first** in a series of PRs that will relocate all
`balanceTx`-related tests to a new module
`Cardano.Write.Tx.BalanceSpec`. This module will **eventually** live
within the `cardano-balance-tx` library.

This PR:
- only relocates `balanceTransactionSpec`.
- creates the new module within the `cardano-wallet` library (for the
moment).
- makes a limited number of exports from the new module to the old
module. (These will eventually be removed.)

## Further work

Further PRs will:

- relocate other `balanceTx`-related tests to the new module.
- relocate the new module to the `cardano-balance-tx` library.

lib/wallet/cardano-wallet.cabal

@@ -921,6 +921,7 @@ test-suite unit
     Cardano.Wallet.Submissions.PrimitivesSpec
     Cardano.Wallet.TokenMetadataSpec
     Cardano.WalletSpec
+    Cardano.Write.Tx.BalanceSpec
     Control.Concurrent.ConciergeSpec
     Control.Monad.UtilSpec
     Data.Function.UtilsSpec