I have some really good news I can pass along to you. We made this process fairly painless. You don't have to even think about Pages.g.cs. This file is dynamically generated based on all the elements contained in all your tests. See here for more details.
Since each test (the .tstest file specifically) actually maintains the definition for the elements it uses/acts on you can literally just move files around and the elements get carried along with it. A new Pages.g.cs will be automatically generated for your new project containing only the elements for the tests contained in that project.
The only "gotcha" you may have to deal with is tests that are used in our Test-as-step feature. Since we do not support using tests across multiple projects, you may have to duplicate your subtests in both projects. Except for this gotcha to watch out for, it literally is as simple as moving files around.
If you have any other questions, concerns or run into problems don't hesitate to reach out to us. We'll be glad to help you through this process.
the Telerik team