PHP Fatal error: Class 'PHPUnit\Framework\TestCase' not found with PHPUnit 6 and PHP 7.0

I am playing with php 7 and phpunit 6. Here is the test I wrote:

<?php declare(strict_types=1);

namespace Test;

use DesignPatterns\Observer\User;
use DesignPatterns\Observer\UserObserver;
use PHPUnit\Framework\TestCase;

class ObserverTest extends TestCase
{
    public function testChangeInUserLeadsToUserObserverBeingNotified()
    {
        $observer = new UserObserver();

        $user = new User();
        $user->attach($observer);

        $user->changeEmail('[email protected]');
        $this->assertCount(1, $observer->getChangedUsers());
    }
}

When I tried to run this test, I got the following error message:

PHP Fatal error:  Class 'PHPUnit\Framework\TestCase' not found in /home/.../.../Test/ObserverTest.php on line 9

I installed PHPUnit with composer, here is my composer.json file content:

{
    "require": {
        "phpunit/phpunit": "^6.0"
    },
    "autoload": {
        "psr-4": {"DesignPatterns\\": "src/"}
    }
}

According to PHPUnit 6 documentation, your tests are now supposed to extends PHPUnit\Framework\TestCase instead of PHPUnit_Framework_TestCase.

I know it's not an issue with autoloading. Actually, if I replace PHPUnit\Framework\TestCase with PHPUnit_Framework_TestCase, it works just fine, but I was wondering why this syntax didn't works.

I tried some research on google, stackoverflow and PHPUnit's github repository, but couldn't find anything.

I am looking forward for your answers,

EDIT

This is how my files are organized:

src/
├── DataMapper
│   ├── StorageAdapter.php
│   ├── UserMapper.php
│   └── User.php
├── Observer
│   ├── UserObserver.php
│   └── User.php
Test/
├── DataMapperTest.php
└── ObserverTest.php