add testcase for get head comment

src/test/java/com/alibaba/druid/bvt/sql/FirstCommentTest.java

@@ -0,0 +1,21 @@
+package com.alibaba.druid.bvt.sql;
+
+import java.util.List;
+
+import org.junit.Assert;
+
+import com.alibaba.druid.sql.dialect.mysql.parser.MySqlLexer;
+import com.alibaba.druid.sql.parser.Lexer;
+
+import junit.framework.TestCase;
+
+public class FirstCommentTest extends TestCase {
+    public void test_first_commnet() throws Exception {
+        String sql = "/*test sql*/select age from user where name = 'xx';";
+        Lexer lexer = new MySqlLexer(sql);
+        lexer.setKeepComments(true);
+        lexer.nextToken();
+        List<String> comments = lexer.readAndResetComments();
+        Assert.assertEquals("/*test sql*/", comments.get(0));
+    }
+}