Apparent problem with @test_warn
It appears that placing @test_warn inside a @testitem block breaks the VSCode/Julia testing. When running a failing test for @test_warn, VSCode tries to open an editor window for Test.jl, which only shows the message "The editor could not be opened because the file was not found."
I think this is essentially a problem in Base... If you look at the implementation of the @test_warn macro here you can see that it just calls the @test macro in turn. So the file location where the test is failing is actually not the location where the @test_warn macro is being called, instead it will report the location that I linked to here. And then it also seems that the default Julia stores filenames from the build bots for standard library locations, so then VS Code is trying to open a file that doesn't exist.
I think the proper way to fix this is to fix @test_warn to correctly report the location of the @test_warn invocation if it fails.
https://github.com/JuliaLang/julia/issues/47033