Revise and simplify test suite.

[jonathanknowles]

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.