Removed redundant include statements from test programs.

The needed header files are already included with SDL.h. Still including them in
the test programs is confusing because it somehow suggests they would be needed.

test/testthread.c

@@ -17,7 +17,6 @@
 #include <signal.h>
 
 #include "SDL.h"
-#include "SDL_thread.h"
 
 static SDL_TLSID tls;
 static int alive = 0;