Fixed memory leak in testthread

(cherry picked from commit dfe6efc)
(cherry picked from commit 224a37f)

test/testthread.c

@@ -19,6 +19,7 @@
 #include "SDL.h"
 
 static SDL_TLSID tls;
+static SDL_Thread *thread = NULL;
 static SDL_atomic_t alive;
 static int testprio = 0;
 
@@ -77,13 +78,13 @@ killed(int sig)
     SDL_Log("Killed with SIGTERM, waiting 5 seconds to exit\n");
     SDL_Delay(5 * 1000);
     SDL_AtomicSet(&alive, 0);
+    SDL_WaitThread(thread, NULL);
     quit(0);
 }
 
 int main(int argc, char *argv[])
 {
     int arg = 1;
-    SDL_Thread *thread;
 
     /* Enable standard application logging */
     SDL_LogSetPriority(SDL_LOG_CATEGORY_APPLICATION, SDL_LOG_PRIORITY_INFO);