I am currently using Xcode 15.2, but I am sure this issue exists on other versions too. If you have any packages made pre Xcode 14.3, i.e. ones without an automatically made test plan, then the following issue happens:
Let's call the package in this example PackageX, and you have with this a corresponding test scheme, called PackageXTests. PackageXTests is a test scheme saved on disk, that is on the test scheme of PackageX, meaning that when you test the package directly, it will run the test scheme.
Now, with test plans becoming the norm, we want to move away from having these test schemes on disk and instead align old packages with new and have each package's tests dictated by its own corresponding test plan, the same way that any newly made package would work.
So we click 'convert to test plan', success we have a test plan, we delete the test scheme from the disk. All appears to be working fine. However, if you now swap branch and come back, the test plan will no longer be connected to that package. It will have reverted back to a test scheme. Which in git shows as a test scheme which it has added (i.e. the git diff has an uncommitted test scheme it has made).
This is not a derived data issue, as I have deleted my derived data and it still occurs. Similarly with cleaning and building.
