You're not really testing the behaviour of ExpressionLanguage. You should rather do something like:

```php
$expressionLanguage = new ExpressionLanguage();

define('expression_language_foo', 'foo');

$this->assertSame('foo', $expressionLanguage->evaluate('constant(expression_language_foo)'));
```

I also suggest to make a constant name more unique, because it could potentially conflict in a constant  defined in another test.