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

I found the answer:

I was excuting my test with this command line:

phpunit Test/ObserverTest.php

PHPUnit is installed globally on my computer, but it's the 5.1.3 version:

phpunit -v

PHPUnit 5.1.3 by Sebastian Bergmann and contributors.

Runtime:       PHP 7.0.13-0ubuntu0.16.04.1 with Xdebug 2.4.0
Configuration: /home/.../.../DesignPatterns/phpunit.xml

And the syntax PHPUnit\Framework\TestCase only works with PHPUnit 6

Now, if I run php vendor/bin/phpunit Test/ObserverTest.php, it works perfectly...