What are tests and who tests what

Testing is checking, and here we have (should have) a clear division between the checker and the checked. It is typical of the IT market to tell customers that they have no competence in analysis, testing, design, that all this will be done by the software supplier. Thus, the following are ordered from the same, single, software supplier: business analysis, requirements analysis, design, implementation, writing acceptance tests and testing. An entity like the Purchaser in essence just watches, and pays as much as the suppliers want (T&M projects are out of control). In effect, the checker and checked are the same entity. Does this make sense?

Source: Czym są testy oraz kto i co testuje – Jarosław Żeliński IT-Consulting