Support for interop unit tests in Foundry

[fainashalts]

At the unit test level, developers will likely wish to test their smart contracts against interop functionality from the perspective of each chain they will be deployed one. A common way to do unit tests in Foundry is to use forking cheatcodes. For more details on forking cheatcodes, see the Foundry documentation.

We should ensure that developers have what they need to efficiently create unit tests for interop functionality.

To Do:

• provide code snippets in the interop developer documentation for how to use vm.mock() to test interop functionality (to mock things like message initiation)
• evaluate whether additional cheat codes need to be added to foundry to support the developers testing interop projects. Record the decision and rationale either way.