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/testsem.c

@@ -15,6 +15,7 @@
 #include <signal.h>
 
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
 #define NUM_THREADS 10
 /* This value should be smaller than the maximum count of the */