There are various reasons you might not want to check CarolineXCTest.swift into sourcecontrol. Notably, if you have your tests folder shared between atbuild and Xcode, you want CarolineXCTest to be linked for Xcode but not for atbuild. To achieve this, you might decline to check it in.
However, if caroline-static-tool encounters a swift sourcefile in the Xcodeproj it can't locate, it crashes. This is the right behavior normally (missing source file is Bad News Bears) but the wrong behavior for CarolineXCTest since it will just be generated during the build.
We resolve this by ignoring the file not found in that particular case, as presumably there are no tests in CarolineXCTest.swift that we are missing (since we own it, and we generate it).
We implement test coverage for this by running the static tool on an Xcode project and deleting the file beforehand. We don't currently use the project for any other xcode-related tests, although we certainly could use it as an integration host if we wanted to.