feat: Upload and visualize test attachments (e.g. snapshot diff images)

Summary

Support uploading and displaying test attachments — such as screenshot diffs from failed snapshot tests — directly in Tuist’s test results UI.

Motivation

Many teams use snapshot testing (e.g. with swift-snapshot-testing) and when a test fails, developers need to compare the reference image, the actual image, and the diff. Currently, building infrastructure to store and visualize these failing snapshot test artifacts is a hassle for users, requiring custom tooling and separate storage solutions.

This came up from a user request:

Is it possible to add uploading diff images by failed test? It will be more convenient to check my test failed.

This is a common need across teams using snapshot testing, and having first-class support in Tuist would significantly improve the developer experience for diagnosing test failures.

Proposed Solution

• Allow test attachments (images, files) to be uploaded alongside test results
• Display attachments (especially images) in the test results dashboard
• Support common snapshot testing patterns: reference image, actual image, and diff image
• Consider integration with xcresult bundles which already contain test attachments

Use Cases

• Snapshot testing: View reference vs. actual vs. diff images for failed visual regression tests
• UI testing: View screenshots captured during XCUITest runs
• General test artifacts: Attach any file generated during test execution for later inspection