In a larger project, I got the following error message:

Visual Studio could not open the file "c:\TestAutomation\BuildVerificationTests.orderedtest" because the file contains tests. A test can be opened only within a test project. Add the file to a test project, add the test project to the open solution, and then try again.

Reason: The solution contains a service model (Service Factory). Right-clicking and selecting Unload Project (on the service model) fixes the problem with not being able to open the ordered test.