Extend testing when provisional extensions are enabled

Could we extend the tests themselves to observe the effects of the macro, for example something like this in`tests/test_ext_headers.c`

```cpp
int provisional_macro(void) {

#ifdef CL_ENABLE_PROVISIONAL_EXTENSIONS
printf("%s is available\n", CL_KHR_COMMAND_BUFFER_EXTENSION_NAME);

#else

#ifdef CL_KHR_COMMAND_BUFFER_EXTENSION_NAME
assert(false && "cl_khr_command_buffer macros should not be defined");
return 1;
#endif // CL_KHR_COMMAND_BUFFER_EXTENSION_NAME

#endif // CL_ENABLE_PROVISIONAL_EXTENSIONS

return 0;
}

```

_Originally posted by @EwanC in https://github.com/KhronosGroup/OpenCL-Headers/pull/268#discussion_r1893840241_

Start in tests/test_ext_headers.c and review the existing provisional-extension tests. Add coverage that observes the macro effects when CL_ENABLE_PROVISIONAL_EXTENSIONS is enabled and verifies the related macros are absent when it is disabled; done means the test suite checks both cases.

Written by the indexing model from the issue text.