Aspect-based refactoring, called aspectualization, involves moving program code that implements cross-cutting concerns into aspects. Such refactoring can improve the maintainability of legacy systems. Long compilation and weave times, and the lack of an appropriate testing methodology are two challenges to the aspectualization of large legacy systems. We propose an iterative test driven approach for creating and introducing aspects. The approach uses mock systems that enable aspect developers to quickly experiment with different pointcuts and advice, and reduce the compile and weave times. The approach also uses weave analysis, regression testing, and code coverage analysis to test the aspects. We developed several tools for unit and integration testing. We demonstrate the test driven approach in the context of large industrial C++ systems, and we provide guidelines for mock system creation.