You might think you were in luck when Xcode offered you an option to migrate to XCTest, but shortly after trying you will discover that this isn't a true migration. Xcode will only migrate your code, it will not update your project to be testable with XCTest. There are still some steps left and unfortunately Apple have not shared any documentation on how to do this.
Note
I have later discovered that it does work as expected, but only if you run the conversion from a project directly. If you try running the conversion from a workspace you will get the problems mentioned above. I have filed this as rdar://16581037 so Apple can fix this in the future. For now, you'll need to follow the below steps.
You can follow these steps to migrate your project to XCTest once you've migrated the code with the automatic converter: