feat: Incorpore scripts, docs and examples to run Unit tests for capacitor plugins

## Feature Request

### Description
Incorpore scripts, docs and examples to run Unit tests for capacitor plugins

### Platform(s)
Android, iOS

### Additional Context
Cordova suffered from the problem that a lot of plugins were (are) not tested, this make them unstable and hard to update without break anything. Capacitor should do better than that, there is currently no documentation about how to run unit tests for plugins in the plugin project.