Activate a workspace test project directly

[simsurace]

A package's test sub-project that is a Pkg workspace member is a complete, already resolved environment whose Manifest is the shared workspace Manifest, located by walking the workspace chain up from the folder. The scratch-env plus TestEnv path cannot serve it: it copies the project into a scratch directory, severing the workspace relationship, then finds no Manifest beside the copy and fails to resolve because the workspace's path-tracked packages are not registered. Activate such a project in place instead; activation is read-only, so it still never writes into the tree.

[davidanthoff]

I have some changes/additions coming, I'll push to this branch, ok?

[davidanthoff]

Thanks — the direction is right: Pkg.test itself (Julia 1.12+) has exactly this branch and runs a [workspace]-member test/ project in place instead of sandboxing it. I pushed a commit on top (30b9ab2) that aligns the details with what Pkg does:

• Gate on workspace membership, not the folder name. Pkg checks testdir in keys(env.workspace); the PR checked only <pkg>/test. That also caught a test/Project.toml with a manifest of its own that devs the package, which Pkg sandboxes — and on Julia < 1.12, where workspaces don't exist, that was the only shape it could catch. The check now goes through Base.base_project, the same thing Pkg consults.
• Instantiate and precompile, as Pkg's workspace branch does. Pkg.instantiate only fetches into the depot when the manifest exists, so a workspace without a resolved manifest is refused with a message rather than resolved into the user's tree. Precompiling here keeps the cache build inside the controller's serialized activation window.
• Case-insensitive path comparison on Windows (the two URIs of a request can disagree about the drive letter) and trailing-separator handling.
• Tests: the membership helper, an end-to-end workspace fixture with an unregistered path-tracked dependency (asserts the tree is untouched and the member project is the active one), and a non-member test/ with its own manifest that must still land in a scratch copy. Plus a CHANGELOG entry.

All of test/test_scratch_env.jl and the other activation-related test files pass on 1.13 here.

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