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/testresample.c

@@ -11,6 +11,7 @@
 */
 
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
 int main(int argc, char **argv)
 {