字符串工具类：subStr增加测试用例、代码示例

hutool-core/src/test/java/cn/hutool/core/text/StrUtilTest.java

@@ -280,6 +280,16 @@ public class StrUtilTest {
 		Assert.assertEquals("ghigh", pre);
 	}
 
+	@Test
+	public void subPreTest() {
+		Assert.assertEquals(StrUtil.subPre(null, 3), null);
+		Assert.assertEquals(StrUtil.subPre("ab", 3), "ab");
+		Assert.assertEquals(StrUtil.subPre("abc", 3), "abc");
+		Assert.assertEquals(StrUtil.subPre("abcd", 3), "abc");
+		Assert.assertEquals(StrUtil.subPre("abcd", -3), "a");
+		Assert.assertEquals(StrUtil.subPre("ab", 3), "ab");
+	}
+
 	@Test
 	public void subByCodePointTest() {
 		// 🤔👍🍓🤔