Tests: Permit fine-grained control over the enabled plugins

[fendor]

From: #3647 (comment)

It should be possible to specify exactly which plugins are enabled during a test run.

[michaelpj]

I think you can kind of do this with runServerWithSession no? Or do those still have the ghcide plugins?

[fendor]

Yeah, they do have wired in plugins, e.g. NotificationHandlers likely have to be enabled for anything to work. But code lenses and code actions are likely optional. It is probably a minor refactoring.