Provide support and guidance for testing an extension against its parent package

When an extension is not developed in the same package that it is shipped with (like standalone extensions are), it can be challenging to test that the extension is compatible with its parent package. For example, if a DevTools extension performs an expression evaluation on a library from the parent package within the context of a running app, there is not an easy way to verify that these expression evaluations are valid in a testing environment.

To support this, we should expose some integration test helpers that support running a live app and connecting it to the simulated DevTools environment. We have some of this infrastructure in devtools_app already, but we would need to move this to devtools_app_shared.