I would like to write automated tests for a Safari extension with Selenium, but I don't find any documentation how to use the new Safari built-in WebDriver to access an installed Safari extension.
Currently you cannot even load Safari extensions with Selenium anymore, atleast since Selenium 3.x.
If I was building a Safari extension I would write a lot of unit and component test. Then either test the wiring (e.g. end-to-end Smoke tests) manual or with something like Sikuli.
Even the Safari extension developer documentation does not speak about testing. Wondering you shouldn't stay away from this platform.