Implement Snapshot testing

[DominikBucher12]

I was thinking about how to properly test the rich text kit implementation, unit tests and integration tests are great to mimic the behaviour, but if we want to be close to 100% accurate, we should implement Snapshot tests in order to be 100% sure that everything behaves correctly and we are on a right track with setting the attributes.

I was thinking we could implement https://github.com/uber/ios-snapshot-test-case/ as well as https://github.com/pointfreeco/swift-snapshot-testing

Or we can write some minimal solution ourselves. Things, thoughts, ideas @danielsaidi ?