defrag: Fix unittest logic.

We've linked the size of hash with trackers. Thus calling DefragInit()
after setting the configuration variable is more logic.

src/defrag.c

@@ -2411,14 +2411,14 @@ DefragTimeoutTest(void)
     int ret = 0;
     DefragContext *dc = NULL;
 
-    DefragInit();
-
     /* Setup a small numberr of trackers. */
     if (ConfSet("defrag.trackers", "16", 1) != 1) {
         printf("ConfSet failed: ");
         goto end;
     }
 
+    DefragInit();
+
     dc = DefragContextNew();
     if (dc == NULL)
         goto end;