Fixed undefined ImageValidator:: property when uploading an image during functional tests
bug #11111 Fixed undefined ImageValidator::$suffices property when uploading an image during functional tests (OwlyCode)

This PR was squashed before being merged into the 2.5 branch (closes #11111).

Discussion
----------

Fixed undefined ImageValidator::$suffices property when uploading an image during functional tests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11110
| License       | MIT
| Doc PR        |

This will make the FileValidator::$suffices attribute protected so ImageValidator or any inherited class can access it. It fixes #11110 in the most simple way possible.

Commits
-------

859f34e Fixed undefined ImageValidator:: property when uploading an image during functional tests