limit pinning tests to CPM-downloaded projects

[jameslamb]

Description

Modifies pinning tests from https://github.com/rapidsai/rapids-cmake/pull/530:

• to only test projects that were downloaded by CPM (e.g. ignoring the fmt that might already exist in the build environment)
• to echo out pinned_versions.json and versions.json in logs from failed tests, to make debugging faster

Notes for Reviewers

592 proposes some testing changes that aren't specific to the goals of that PR.

Since that PR might be stuck for a bit (https://github.com/rapidsai/build-planning/issues/56#issuecomment-2086525874), this proposes pulling those out into a separate PR:

• so that other changes in this project benefit from them
• to shrink the diff of #592 and therefore the risk of merge conflicts

How I tested this

Pushed a commit with the new test error message content changes but keeping fmt in the failing tests, to confirm that the expected tests failed.

got the expected outputs (click me)

```text The following tests FAILED: 698 - cpm_generate_pins-nested-makefile (Failed) 700 - cpm_generate_pins-nested-ninja (Failed) 702 - cpm_generate_pins-nested-ninja_multi-config (Failed) 722 - cpm_generate_pins-simple-makefile (Failed) 724 - cpm_generate_pins-simple-ninja (Failed) 726 - cpm_generate_pins-simple-ninja_multi-config (Failed) ``` And they failed in the expected way more informative logs! ```text CMake Error at CMakeLists.txt:51 (message): pinned fmt tag (10.2.1) should differ compared to baseline 10.2.1 pinned_versions.json: { "always_download" : true, "git_shallow" : false, "git_tag" : "${version}", "git_url" : "https://github.com/fmtlib/fmt.git", "version" : "10.2.1" } versions.json: { "git_tag" : "${version}", "git_url" : "https://github.com/fmtlib/fmt.git", "version" : "10.2.1" } ``` ([build link](https://github.com/rapidsai/rapids-cmake/actions/runs/8837613213/job/24267079292?pr=592))

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [x] The cmake-format.json is up to date with these changes.
• [ ] I have added new files under rapids-cmake/
  • [ ] I have added include guards (include_guard(GLOBAL))
  • [ ] I have added the associated docs/ rst file and update the api.rst

[robertmaynard]

/merge