Implement full end-to-end testing

[claucambra]

At present, the macOS virtual files client's automated test suite is limited.

NextcloudFileProviderKit features an extensive test suite that covers the expected behaviour of many units within the package, using extensive mocking of a real Nextcloud server instance and of the types of tasks the system's fileproviderd process would require a File Provider Extension to perform. However:

• There is no testing against a real server
• There is no testing using the complete File Provider app extension bundled into the client

This task requires you to implement proper end-to-end tests using Apple's File Provider testing APIs. Ideally, these tests should check the expected behaviour against a real server instance that is spun up and shut down on demand in a CI/CD environment in GitHub.