(chore) Infrastructure: Add flutter golden tests.

[hastebrot]

Add screenshot tests that use Flutter Gold (which is integrated into Flutter as part of flutter_test) and Golden Toolkit (which contains APIs and utilities to provide UI regression tests).

• Run all tests: flutter test.
• Run all tests and update golden screenshots: flutter test --update-goldens.

This functionality is also useful to quickly create screenshots of UI components to be used in pull request descriptions.

[hastebrot]

I've rebased the pull request. Ready to go.