Since we all love theoretical knowledge so much, we would also like to comment on the responsibility of the respective tests.
The TestDaF is subject to university supervision, which is also part of the board of G.A.S.T. . What is does G.A.S.T. yet again? G.A.S.T is the Society for Academic Study Preparation and Test Development (g.a.s.t.). Among it, you will find institutes such as the HRK, the DAAD, the Goethe Institute, the Fernuni Hagen, etc. The exams are regulated by the framework regulations for German language exams for studying at German universities (RO-DT). As you can see, everything is very professional and safe.
Let’s take a look at telc gGmbH. This is a full member of the Association of Language Testers in Europe (ALTE) since 1995. This is an association of language test providers. telc exams are based on the ALTE quality criteria and are constantly checked. In the ALTE are e.g. the Institute for Basque, Bulgarian, Catalan, telc, the Goethe Institute, etc.). You can find the full list of members here.