fix test_thread_yield fail bug in multiple utest loop case.
fix test_thread_yield fail bug in multiple utest loop case.

Signed-off-by: fan.ji <d552fe998203f9031e59b2ce480e116c93ff0dfb@weiheng-tech.com>

RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread,RT-Thread/rt-thread