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

…4146)

## Issue

ADP-3171
Follow-on from #4145

## Description

This PR relocates `distributeSurplusSpec` to
`Cardano.Write.Tx.BalanceSpec`.

## Context

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

## Further work

Further PRs will:

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