[Finder] Unified tests

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -


Tests for `Finder` are very difficult to read because they contain calls to:

    sys_get_temp_dir() . '/symfony2_finder/'

This improved version simplifies adding and removing new dirs and files.