doctrine phpcr: add File::getSize test

This PR is to add a test for the ``File::getSize`` method, to ensure it works as expected.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A