[Contribute to AppFlowy] Unit Test: How to Write a Test

The part related to writing test needs some more documentation. As a newcomer, I feel the part related to writing test is really important to get started to contribute to AppFlowy. It could help us completing the checklist when submiting the first pr as well as speeding up the process to get familiar with the codebase. In addition, it can also help to increase the test coverage if more people are able to start with writing some testcases as a way to start contributing to AppFlowy.