tests first.c: fix calculation of sleep timeout on Windows

Not converting to double caused small timeouts to be skipped.

tests/libtest/first.c

@@ -56,7 +56,7 @@ int select_wrapper(int nfds, fd_set *rd, fd_set *wr, fd_set *exc,
    * select() can not be used to sleep without a single fd_set.
    */
   if(!nfds) {
-    Sleep(1000*tv->tv_sec + tv->tv_usec/1000);
+    Sleep((1000*tv->tv_sec) + (DWORD)(((double)tv->tv_usec)/1000.0));
     return 0;
   }
 #endif