Make source_with_oneline_annotations.php valid PHP

Currently `Tests/_files/source_with_oneline_annotations.php` isn't valid PHP since the interface is missing an identifier.

```
$ php -l Tests/_files/source_with_oneline_annotations.php 
PHP Parse error:  syntax error, unexpected '{', expecting identifier (T_STRING) in Tests/_files/source_with_oneline_annotations.php on line 4
```