Create extension-level unit and integration tests

[lionel-]

We currently test air from Rust with both unit tests and protocol-level tests with a dummy client. However we should also have integration tests in the VS Code extension:

• Some features are provided by the extension and should be tested on their own, such as settings synchronisation.

• We should also add some integration tests for major features. These will also be run by the Positron test suite to make sure everything is in working order. This will be important to ensure quality as Air increasingly becomes part of the R development experience in Positron.