Integrate Asset Test Utilities for AssetHubWestend

[raymondkfcheung]

This issue continues the work from #7952 by migrating exchange_asset tests to the AssetHubWestend runtime module using asset-test-utils.

Changes

• Moves existing exchange_asset tests from the integration test suite to unit tests under AssetHubWestend.
• Utilises asset-test-utils to avoid redundant test logic and leverage predefined test cases.
• Ensures test coverage aligns with expected cross-chain interactions where necessary.

Context

As suggested in this discussion, these tests primarily interact with AssetHubWestend, making them suitable for inclusion as unit tests rather than integration tests. The migration reduces test duplication while improving maintainability.

Draft

#8074

[haider-rs]

@raymondkfcheung i am picking up this issue.

Update: currently busy with life if anyone wanna take it up.

[Kanasjnr]

Is this issue still available to work on?

[PanGan21]

Hey @raymondkfcheung since it is contributors friendly issue I gave it a try here: #10721
Let me know if it resolves the issue or something else is needed.