Add more unit tests for the class of `ConnectionSavepointManager`

[FlyingZC]

Hi community, This issue is for #20305

Aim

Add more unit tests for the class of org.apache.shardingsphere.transaction.ConnectionSavepointManager.

Basic Qualifications

• Java
• Maven
• Transaction

Detail

Maybe you can mock the connection and assert the results of setSavepoint,rollbackToSavepoint... methods.

private Connection mockConnection() throws SQLException {
      Connection result = mock(Connection.class);
      when(result.unwrap(JdbcConnection.class)).thenReturn(mock(JdbcConnection.class));
      return result;
}