优化ThreadUtil.safeSleep，使用System.nanoTime()

hutool-core/src/test/java/cn/hutool/core/thread/ThreadUtilTest.java

@@ -1,5 +1,6 @@
 package cn.hutool.core.thread;
 
+import cn.hutool.core.util.RandomUtil;
 import org.junit.Assert;
 import org.junit.Test;
 
@@ -11,4 +12,13 @@ public class ThreadUtilTest {
 
 		ThreadUtil.execute(() -> Assert.assertTrue(isValid));
 	}
+
+	@Test
+	public void safeSleepTest() {
+		final long sleepMillis = RandomUtil.randomLong(1, 1000);
+		// 随机sleep时长，确保sleep时间足够
+		final long l = System.currentTimeMillis();
+		ThreadUtil.safeSleep(sleepMillis);
+		Assert.assertTrue(System.currentTimeMillis() - l >= sleepMillis);
+	}
 }