From T231358, 062e067d4964c2854c33821fb5522cf578dc4f89 for MinervaNeue made a ResourceLoaderImageModule invalid since it has an image pointing to a non existent file:
```
lang=diff
+ "userSandbox": "resources/resources/skins.minerva.personalMenu.icons/userSandbox.svg"
^^^^^^^^^^^^^^^^^^^
- "userSandbox": "resources/skins.minerva.mainMenu.icons/userSandbox.svg"
```
MediaWiki has structure tests to validate resources file existence but it does not have any such tests for ResourceLoaderImageModule ones.
`tests/phpunit/structure/ResourcesTest.php` only look for modules being an instance of `ResourceLoaderFileModule`, while `ResourceLoaderImageModule` descend directly from `ResourceLoaderModule`.
We thus need an extra structure test in ResourcesTest.php to validate that images referred to by a `ResourceLoaderImageModule` are actually existing.