Unit testing framework for actions

[nrc]

I would like to be able to test the various actions the RLS can perform without having to create a project, start the server, do the initialisation, etc. (i.e., an integration test). This is currently difficult because we have a lot of heavyweight context. I believe that if we can mock the ActionContext, unit testing should then be straightforward. However, mocking the ActionContext includes mocking the VFS, configuration, IO, the analysis data, and the build manager. Which will be interesting!