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.
I agree with @Tobion. there is no point changing the visibility. Fixing the code reading it is better