Testing yii2 using code

Question

I am writing a simple test case. I installed the code using composer. My test cases are in the root folder inside the 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 the browser, I get the following error

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

This file is called yiisoft \ yii2-codeception \ TestCase.php in my project

What am I doing wrong? Can anyone help?