Add xcm-simulator

[evilrobot-01]

Adds a xcm-simulator, based on the example within the Polkadot repository, with Trappist-specific use cases as tests. Each closure scope in each test can be thought of as a block on the corresponding chain, with messages being dispatched to target chains via the mock message queue as it goes out of scope.

It effectively translates the integration tests in #76 into Rust with the following main benefits:

• enables interactive debugging with breakpoints, allowing stepping through code to see exactly what is going on as messages are processed across chains
• uses existing runtime config as much as possible, simulating actual runtime interactions as best as possible
• shows xcm tracing output in test console for further additional insight

The tests can simply be run from the command line as follows, assuming one is in the root folder of the project, but the real value is running the tests with a debugger and breakpoints to explore how the instructions are handled by the xcm-executor. Note also that the crate is excluded from the workspace, effectively making it 'opt-in'.

cd xcm-simulator; cargo test; cd ..

Potential next steps:

• implement a helper function for determining fees, better demonstrating the relationships between xcm weights/fees
• add GH action to run when creating PRs, ensuring no regressions in cross-chain functionality

Any other suggestions for improvement welcome!

[stiiifff]

Great work @evilrobot-01, this is a super valuable addition to the repo.
Can you add instructions in the README on how to run the tests ?
Also, as you mentioned it, it would be nice to run these as part of a CI task.

[evilrobot-01]

Updated the PR description with a crude way to run the tests. Will update the readme and add a separate GH action for now, which can always be incorporated into the main CI flow as/when required.

[evilrobot-01]

Updated the readme, thanks to additional information from @bernardoaraujor 🙌, and then also added a separate CI workflow which outputs the test results as an artifact. I had hoped to also include the xcm-executor tracing output as further demonstration of what is available from the simulations, but nextest doesnt currently support this for passing tests so I have opened a feature request issue there.

PS - nextest used in CI as it allows converting results into junit format for nicer reporting using test-reporter. Happy to hear of any better alternatives, haven't explored the GH test reporting landscape for a few months.

[stiiifff]

@evilrobot-01 Super cool, I love the test report. Should we add a badge to the README with the latest "XCM tests" status ?

[evilrobot-01]

Sure, any preference on location? Could add one for the build at the same time.