Replace TestSuite factory with TestCaseFactory

Currently, all test cases must be instantiated at registration. This as a drawback with regards to memory consumption, and complexify significatively the interface as care is required for dealing with memory management. It as also made implementation of parametrized test cases complex. Instead of providing direct access to TestCase, the user should only manipulate its factory (get Test meta data, run the test case...). This will help hiding most the implementation detail of TestCase.