Add XCResult attachments for missing snapshots

[jflan-dd]

It would be useful if snapshot tests added an XCAttachment for snapshot tests with missing reference images.

We have a step in CI that checks the XCResult file for updated snapshot images and will automatically create a PR with the updated images for the engineer to review and merge into their feature branch if the changes are good.

It's been working great, but it doesn't handle new snapshot tests that don't have a reference image on disk. Ideally engineers should capture a snapshot locally and commit that for every new snapshot test, but it's come up multiple times that being able to get the new snapshot from the XCResult file would be useful.