[Finder] Unified tests
merged branch gajdaw/finder_unified_tests (PR #7197)

This PR was merged into the 2.2 branch.

Commits
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a59e5e4 [Finder] Unified tests

Discussion
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[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.