yii2 testing using codeception

I am writing a simple test case. I installed codeception using composer. My test cases are located in root folder inde test case folder

when i try to run the following code

namespace app\tests\testcases;

use yii\codeception\TestCase;

class SomeMyTest extends TestCase
{

public function testOne()
{
    echo "here";
}



}

when i try to run the code in a browser i get the following error

 Fatal error: Class 'yii\codeception\TestCase' not found 

There is this file called as yiisoft\yii2-codeception\TestCase.php in my project

What am i doing wrong? Can anyone help.?