Platform needs a test framework

There is an internal test framework in iTasks (https://gitlab.science.ru.nl/clean-and-itasks/iTasks-SDK/blob/master/Libraries/iTasks/Internal/Test/Definition.dcl), and there is another one here: https://github.com/mklinik/clean-TestFramework. mklinik's test framework supports pure tests, IO tests, Task tests and Gast properties. The hierarchical organization of Bas' framework is better, with test suites and everything.

It would be nice if a test framework was available in clean platform, without the iTasks specific code. The iTasks specific layer should be optional. Ideally, all additional features like IO, Gast, iTasks should be optional.

We should combine these two frameworks where it makes sense and put it in Platform.