[Finder] [Iterator] Make the tests less fragile

Modified the Iterator tests so that they assert only on what's actually being tested. In particular, the tests were checking the order of the files [that were created virtually simultaneously] after sorting them by date. This change is a part of the effort to make Symfony successfully pass tests on HHVM

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [no]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [none]
| License       | MIT
| Doc PR        | [none]
``is_callable`` is useless. A callable will never be equal to any of these constants