Add SDL_main.h includes to tests

.. including cmake/test/*.c, whatever that does

(this is needed now because SDL.h doesn't include SDL_main.h anymore)

test/testshader.c

@@ -12,6 +12,7 @@
 /* This is a simple example of using GLSL shaders with SDL */
 
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
 #include <stdlib.h>