PHPUnit: Failed opening required `PHPUnit_Extensions_Story_TestCase.php`

I installed PHPUnit with:

wget https://phar.phpunit.de/phpunit.phar
chmod +x phpunit.phar
mv phpunit.phar /usr/local/bin/phpunit

Trying to run a simple test, but getting:

Fatal error: require_once(): Failed opening required 'PHPUnit_Extensions_Story_TestCase.php'

How do I install PHPUnit_Extensions_Story_TestCase?

The test is simply:

class TestFunctions extends PHPUnit_Framework_TestCase {
    public function test_str() {
        $this->assertEquals('foo', 'bar');
    }
}