> And I can't even call any tests because I can't extend a non existant class, there's no objects in here!.

I'm gonna preach for the church of "away with unit tests": so what? You don't want to test code. Code is not what the user care about.

You want to test behavior. And for this you don't need to extend classes. You should not have to rewrite any code from your application. Like you say: write tests at the seam. Automate user inputs and checking outputs.

A good test suite should not need to be rewritten during refactoring. It should even allow you to change language or replace for an off-the-shelf solution you did not write. If a unit test suite does not allow that, unit tests are an impediment. And I don't care about the "test pyramid", like the IRL ones, it is just a relic; from an age when launching all your tests in parallel was unfathomable.