R: Make it possible to write integration tests that can access RMAIN, for testing callbacks into Rust

Currently, the Amalthea R integration tests cover a variety of scenarios that involve calling Rust from R, but scenarios wherein R calls Rust to e.g. trigger opening a comm (say, for a connection, data explorer, or data viewer) can't be performed under integration tests since they run in a context wherein there is no RMAIN object to handle the request.

We can test how these scenarios work via other means in some cases; for example, the data explorer test manually open comms to check various features. However, it'd make testing more cohesive and representative of real-world interactions if our test environment more closely resembled the real runtime environment.

Start by locating the Amalthea R integration-test harness and the runtime setup that provides RMAIN. Compare it with the data explorer test's manual comm handling; done means integration tests can exercise R-to-Rust callbacks that open comms through RMAIN in a runtime-like context.