[KYUUBI #4148] Update class name of MySQL 8 JDBC driver used in kyuubi-server tests

### _Why are the changes needed?_

Prefer MySQL JDBC driver's new class name `com.mysql.cj.jdbc.Driver` in testing.
To eliminate this warning message in kyuubi-server‘s test runs:
```
[INFO] --- scalatest-maven-plugin:2.0.2:test (test)  kyuubi-server_2.12 ---
Discovery starting.
Loading class `com.mysql.jdbc.Driver'. This is deprecated. The new driver class is `com.mysql.cj.jdbc.Driver'. The driver is automatically registered via the SPI and manual loading of the driver class is generally unnecessary.
```

### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible

- [ ] Add screenshots for manual tests if appropriate

- [x] [Run test](https://kyuubi.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request

Closes #4148 from bowenliang123/test-mysql-driver-class.

Closes #4148

70b1907a [liangbowen] use `com.mysql.cj.jdbc.Driver` for driver class name in MySQLJDBCTestHelper

Authored-by: liangbowen <liangbowen@gf.com.cn>
Signed-off-by: Cheng Pan <chengpan@apache.org>

kyuubi-server/src/test/scala/org/apache/kyuubi/server/mysql/MySQLJDBCTestHelper.scala

@@ -22,7 +22,7 @@ import org.apache.kyuubi.operation.JDBCTestHelper
 
 trait MySQLJDBCTestHelper extends JDBCTestHelper {
 
-  override def jdbcDriverClass: String = "com.mysql.jdbc.Driver"
+  override def jdbcDriverClass: String = "com.mysql.cj.jdbc.Driver"
 
   protected lazy val user: String = Utils.currentUser