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/testsurround.c

@@ -12,6 +12,7 @@
 
 /* Program to test surround sound audio channels */
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
 static int total_channels;
 static int active_channel;