What is the best way to test a program?

Question

Software testing

Hello.
There is a developed program, which consists of modules. All modules are covered to some extent by unit tests. One module can process information, do some calculations and pass it to the input of another module. That is, it turns out that the modules are in some dependence.
The program is transferred to the customer and he wants to develop his modules. In this regard, the question arose: what can be done so that the customer finalizes the program and does not break the existing system with his new modules?
There is only one big test in my head (but it is expensive in terms of time and keeping the tests up to date), but maybe there are some other options?

Answer 1

Force yourself and the customer to use TDD
Test maintenance = software stability
If the customer does not need stability, then do not care at all