Given that for each exam there is assumed knowledge from previous exams (so for Paper FM there is some assumed knowledge from Paper MA), the idea of the short tests on the ACCA website is to check that you remember the relevant knowledge from previous exams before you start studying for the next exam.
Most of the tests are quite good (although all necessary assumed knowledge in Paper FM is explained again in my free Paper FM lectures anyway).
You can find them linked from the ‘study support resources’ for the relevant paper on the ACCA website.