Add tests for Modelix extensions

[ftomassetti]

These tests would require:

• Start a Model Server, containing projects and modules
• Start MPSServer
• Ask MPSServer to checkout stuff
• Verify stuff is checked out

The challenging part would be starting a Model Server with some initial content. Would we create this manually and then use the in-memory dump/load mechanism of the model-server?

[ftomassetti]

We could have a manual procedure to:

• Start the model server
• Copy some examples there
• Dump the content of the model server

This would need to be repeated when the model server format changes.