Improve XCM integration tests

[ebma]

This is a follow-up task to PR #290.

We identified that the integration tests for testing relay-chain transfers via XCM can be improved. We originally adopted these integration tests from Acala, but they added some improvements in the meantime, for example, more flexible fee calculations for asset-hub transfers, see this file, that would make us a little less dependent on hard-coded values.

TODO

We should have another look at Acala's integration tests for relay-chain transfers and try to adopt the latest best-practices.

[ebma]

@pendulum-chain/product I created this very technical ticket. It's more about refactoring and we don't get any new features out of this. It's also not fixing a bug, so not critical. Just nice to have eventually.

[vadaynujra]

@ebma based on your comment I assume this is not required to be completed before this ticket. From an epic pov, would this be more about improving our QA process in general or having a better XCM overview / knowledge?

[ebma]

I'd say this is more about improving the QA process. We could catch some more issues with improving the integration tests. While doing so we might also gain a little extra XCM knowledge but that's more a minor side effect I'd say.

[prayagd]

@ebma Moving this to icebox based on your comment as this nice to have