[VS Incentives]: Revisit decision to export `SetVolume` for testing purposes

[AlpinYukseloglu]

Background

For the sake of simplifying volume split incentive testing, setVolume was exported as a public keeper method in #6366. While this allowed us to avoid significant amounts of confusing test setup, exporting internal functions for testing purposes feels like code smell that should be revisited once the feature is complete.

Suggested Design

• Consider creating a test helper that takes in a target volume amount and gets the pool to that state using only message level functions e.g. LPing and swaps

Acceptance Criteria

• All existing tests pass and volume splitting functionality remains well tested

[p0mvn]

I think we are so deep into relying on this helper everywhere now, making the refactor be not worthwhile in terms of time