The test_headers.c test is designed to make sure that the various cl_typen types work and conform to expectation for recent versions of cl_platform.h. Conforming to these expectations make use of these types practical for developers writing portable code.
The various tests ending in .h.c are there to verify that the various OpenCL headers can compile stand alone. That is to ensure that they may be used a la carte. This provides developers a lifeline in the case that some unneeded part of OpenCL (e.g. cl/gl sharing) brings in a pile of symbols (e.g. all of OpenGL) that collides with other headers needed by the application. It is also poor form to require headers to be included in a particular order, especially if multiple systems require they be included in mutually incompatible order. So, here we require that each header can be used standalone so that the order is irrelevant.
We also check to make sure that the headers don't cause spurious warnings. These tests are intended to be compiled using the most stringent compiler flags available for the platform, within reason. All warnings should be errors and extra warnings that it is expected developers are likely to use should be turned on.