Add integration tests with rust-analyzer

[ethowitz]

I'd love to add integration tests that somehow exercise the integration points with rust-analyzer. I'm thinking something like:

• Invoke the rust-analyzer binary with a configuration that points to cargo-subspace for workspace discovery and flycheck
• Send the appropriate LSP request to rust-analyzer to "open a file," which should prompt it to invoke cargo-subspace
• Assert that cargo-subspace was invoked with the correct arguments
• Assert that rust-analyzer accepted our JSON output (maybe by checking logs/stdout for errors? is it possible to inspect rust-analyzer's internal crate graph?)