[Infrastructure request] Hermetic snapshot testing

[jverkoey]

From reading https://github.com/mastodon/mastodon-ios/blob/develop/Documentation/Snapshot.md, it looks like the snapshot testing infrastructure requires a functional network connection and is primarily optimized for the App Store image generation process.

This is a feature request to add a test suite that's more optimized for development with the following characteristics:

• Is hermetic (i.e. any network requests are faked out).
• Is sparse (i.e. only a subset of the matrix of possible configurations needs to be tested)
• Can be ran on a specific screen (i.e. if you're testing visual changes to a given screen, it's fast to iterate / validate changes across many configurations)
• Bonus: runs automatically as part of the Pull Request workflow.

Downside to the above is it would require checking in screenshots somewhere. This is typically pretty contentious, so I'm filing this feature request mainly for the purposes of kicking off a discussion to see if it's worthwhile pursuing :)