[ADP-3272] Clarify `Arbitrary` instance for `PartialTx`.

[jonathanknowles]

Issue

ADP-3272

Description

This PR clarifies the Arbitrary instance for PartialTx, making the following small improvements:

• Use of explicit field names when constructing a PartialTx:
  • timelockKeyWitnessCounts = mempty is (arguably) more self-documenting than mempty as a positional argument.
• Use of consistent terminology:
  • extraUTxO matches the field name used in PartialTx.
• Clearer data flow:
  • It's (arguably) clearer that the extraUTxO is generated exclusively from the result of generating a tx.

[jonathanknowles]

Merging manually, as:

• This PR only affects the unit test suite for cardano-balance-tx.
• This test suite has been observed to pass locally and on Buildkite (see https://buildkite.com/cardano-foundation/cardano-wallet/builds/4395).
• The branch for this PR is based on the tip of master.