There are a number of mock object libraries out there (mockery, shmock, phpunit's mock syntax, etc). This just looks like another builder syntax for making mocks / stubs with some fanciness around class extension - what's the benefit?
Also, how often is it useful to define a class at test time that is initialized by production code (from the section I Still Have Serious Dependency Issues!). This seems like an unlikely use case.
1) To avoid the complexity of existing frameworks.
2) To be able to quickly and easily test code with various kinds of dependencies.
It is predominately designed to test a single class in an extremely isolated context, not in the context of a full blown testing framework.
It is designed to be a stupid as possible with the intention of allowing the developer (myself) to define very strict test condition and very fine grain control.
Also, how often is it useful to define a class at test time that is initialized by production code (from the section I Still Have Serious Dependency Issues!). This seems like an unlikely use case.