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/testcustomcursor.c

@@ -16,6 +16,7 @@
 #endif
 
 #include <SDL3/SDL_test_common.h>
+#include <SDL3/SDL_main.h>
 
 /* Stolen from the mailing list */
 /* Creates a new mouse cursor from an XPM */