[Finder] [Iterator] Make the tests less fragile
minor #12028 [Finder] [Iterator] Make the tests less fragile (AlphaStream)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #12028).

Discussion
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[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]

Commits
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10f9135 [Finder] [Iterator] Make the tests less fragile