Where we can get documentation on how to use PHPUnit cases for our custom extensions development?

Currently, there is support for Unit Tests on a per module basis, so you can include unit tests in your module. While this is not explicitly documented right now you can look at any core module as an example (e.g., https://github.com/magento/magento2/tree/develop/app/code/Magento/Catalog/Test/Unit).