[Validator] Simplified testing of violations

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

I simplified the assertion of violations in preparation of a replacement PR for #7276.
Sorry about that, but that's a wrong change coming from fabbot/PHP-CS-Fixer. That should be reverted.