refactor sql parser about the desc and explain parts of the code

src/test/java/com/alibaba/druid/bvt/bug/Issue1645.java

@@ -11,8 +11,7 @@ public class Issue1645 extends TestCase {
     public void test_for_issue() throws Exception {
         String sql = "explain extended select * from foo";
         String formatedSql = SQLUtils.format(sql, JdbcConstants.MYSQL);
-        assertEquals("EXPLAIN extended\n" +
-                "SELECT *\n" +
+        assertEquals("EXPLAIN extended SELECT *\n" +
                 "FROM foo", formatedSql);
     }
 }

src/test/java/com/alibaba/druid/bvt/sql/cobar/DALParserTest.java

@@ -18,6 +18,7 @@
  */
 package com.alibaba.druid.bvt.sql.cobar;
 
+import com.alibaba.druid.sql.ast.SQLStatement;
 import com.alibaba.druid.sql.dialect.mysql.ast.statement.MySqlExplainStatement;
 import junit.framework.TestCase;
 
@@ -81,7 +82,7 @@ public class DALParserTest extends TestCase {
     public void testdesc() throws Exception {
         String sql = "desc tb1";
         MySqlStatementParser parser = new MySqlStatementParser(sql);
-        MySqlExplainStatement desc = parser.parseDescribe();
+        SQLStatement desc = parser.parseDescribe();
         parser.match(Token.EOF);
         String output = SQLUtils.toMySqlString(desc);
         Assert.assertEquals("DESC tb1", output);
@@ -90,7 +91,7 @@ public class DALParserTest extends TestCase {
     public void testdesc_1() throws Exception {
         String sql = "desc db.tb1";
         MySqlStatementParser parser = new MySqlStatementParser(sql);
-        MySqlExplainStatement desc = parser.parseDescribe();
+        SQLStatement desc = parser.parseDescribe();
         parser.match(Token.EOF);
         String output = SQLUtils.toMySqlString(desc);
         Assert.assertEquals("DESC db.tb1", output);