Ctest is a wrapper around the Unity C testing framework. It provides a main function that will automagically run all the tests cases you define with the provided TEST(test_function) macro.
When you use the TEST(test_function) macro it will append a function pointer of your test case to a specific section in memory (.data.test_cases) at link time.
Then, the main function will loop through this section of memory, passing the function pointer to the Unity test framework to run the test.