How to test custom implemented Drush Commands ? Are there any Open Source Examples for Drush Commands with Unit Tests or something like that ?
There are a few test traits from drush package, that you can re-use.
As an example check the migration_tools module. It contains a few PHPUnit tests for drush commands:
In addition you can have a look at tests inside the drush package.
As @cilefen mentioned in the comment, there is a documentation section about unit tests at drush github page.