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/testthread.c

@@ -16,6 +16,7 @@
 #include <signal.h>
 
 #include <SDL3/SDL.h>
+#include <SDL3/SDL_main.h>
 
 static SDL_TLSID tls;
 static int alive = 0;