Writing unit tests for a method that calls static methods from another class
You can't mock static calls directly.

A relatively easy way to fix this is what core uses in quite some places to mock functions that haven't been converted to objects/services yet. You create a method in your own class that does nothing but call the static method.

Then for your test, you can either mock your own object and only mock that method or create a test subclass that overrides that method and return something else.