tests: Add a failing test case for OpenImageOptional with oci-archive

[cgwalters]

Prep for fixing it.

[gemini-code-assist]

Summary of Changes

Hello @cgwalters, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a new failing test case to pinpoint and expose an existing bug related to how the OpenImageOptional function handles non-existent oci-archive images. This is a preparatory step to ensure the bug is properly captured before a fix is implemented.

Highlights

• New Dependency: Added the tempfile crate as a new dependency in Cargo.toml to facilitate the creation of temporary directories, primarily for testing purposes.
• Test Utility Function: Introduced a new helper function check_skopeo within the test module of src/imageproxy.rs. This function uses OnceLock to efficiently check for the presence of the skopeo command-line tool, allowing tests to conditionally run based on its availability.
• Failing Test Case for OpenImageOptional: Added a new asynchronous test test_open_optional in src/imageproxy.rs. This test is specifically designed to expose a bug (linked to coreos/rpm-ostree/issues/5442) where ImageProxy::open_image_optional is expected to return None when given a path to a non-existent oci-archive image. The test is currently marked with #[ignore] because it's intended to fail, highlighting the current incorrect behavior, and will pass once the underlying bug is fixed.

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[gemini-code-assist]

Code Review

This pull request adds a new test case for OpenImageOptional with oci-archive. The review focuses on improving the robustness of the new test and dependency management.