add FileUtilTest test case (#672)

hippo4j-common/src/test/java/cn/hippo4j/common/toolkit/FileUtilTest.java

@@ -27,4 +27,15 @@ public class FileUtilTest {
         String contentByFileUtil = FileUtil.readUtf8String(testFilePath);
         Assert.notEmpty(contentByFileUtil);
     }
+
+    @Test
+    public void assertReadUtf8String2() {
+        String linebreaks = System.getProperty("line.separator");
+        String testText = "abcd简体繁体\uD83D\uDE04\uD83D\uDD25& *" + linebreaks +
+                "second line" + linebreaks +
+                "empty line next" + linebreaks;
+        String testFilePath = "test/test_utf8.txt";
+        String contentByFileUtil = FileUtil.readUtf8String(testFilePath);
+        Assert.isTrue(testText.equals(contentByFileUtil));
+    }
 }