[WIP] Add first atoum tests

Adding tests
Where is the `when` and the `then`?
`string` and `isEqualTo` are part of the `then` (the postcondition). The test execution (invocation) is the `when`. I propose the following rewrite:
```php
    ->when($result = $compiler->visit($op))
    ->then
        ->string($result)
            ->isEqualTo(…);
```