DanXi Unit test and Continuous Integration

As the app scales, we need to simplify production process. Currently we have CI for iOS builds, however this is not enough.

Every time we update the engine, Skia Shader Cache (SKSL) needs to be regenerated. Every time we make significant changes to UI, screenshots need to be regenerated and uploaded to stores and our webpage (screenshots on our webpage hasn't been updated for a century!). Both actions are time-consuming but can be automated through unit test and CI. Unit test also reduces the possibility of commits breaking certain functions (especially when the function only breaks on Material or Cupertino). I believe it is about time we automate these things.

TODO:

[ ] Create fake repo for unit tests. These repos return demo data that can be used to test features and generate screenshots without having to actually login and connect to servers. For example, demo UIS login repo could return "DanXi-Dev" as username so screenshots won't expose developers' private info; demo FDUHole repo could return certain select holes for screenshot generation.
[ ] Create UI tests Since our API isn't stable, our unit test mainly tests the UI of each unit. We aim to ensure that no assertion or error is thrown during this process, so that our UI has no hidden issues. SKSL could also be generated during this process to further automate iOS deployment.
[ ] Modify CI system to automatically run UI tests for each release and update screenshots (or even generate changelogs) as needed