TestItemRunner should skip child directories that have their own Project.toml that define a package

[NHDaly]

VSCode test running follows this simple rule: for each @testitem we look in the folder, then in the parent folder etc until we find a Project.toml that defines a package, and then the @testitem is part of that package.

The TestItemRunner.jl implementation doesn't follow that philosophy fully, I think what is missing there is that we should by default skip any child folder that has a new Project.toml in it that defines a new package. So in @NHDaly's tree, if someone runs test/runtests.jl, then that should actually not run the @testitems of the sub packages, right?

Originally posted by @davidanthoff in #39 (comment)