Revise and simplify test suite.

## Context

This issue collects together tasks relating to simplifying the test suite for `cardano-coin-selection`.

## Tasks

- [ ] **`TxIn` and `Address` types** (used for testing)

These can be simplified, as the tests only rely on the fact that generated values are unique and can be ordered. (The internal structure of these types is not relevant.)

In particular, the `Show` instance of `TxIn` is very noisy, making output of tests very difficult to read.

- [ ] **`Arbitrary` instances**

Many of these are duplicates of one another, or very similar. We should consider whether or not it would be worth merging similar instances.

Locate the test definitions for the TxIn and Address types and the Arbitrary instances, then run the existing test suite to understand its current output. Simplify the test-only types, reduce noisy TxIn display output, and consolidate duplicate or similar Arbitrary instances while keeping the tests passing.

Written by the indexing model from the issue text.