fix test_thread_yield fail bug in multiple utest loop case.
@@ -259,6 +259,7 @@ static void thread6_entry(void *parameter)
static void test_thread_yield(void)
{
rt_err_t ret_startup = -RT_ERROR;
+ thread5_source = 0;
tid5 = rt_thread_create("thread5",
thread5_entry,
RT_NULL,
