Fixed inconsistent image storage prefix in test data

[mikadamczyk]

🎫 Issue	N/A

Description:

ibexa_image_file holds two rows for the same content field (attribute ID 172, content 54) with different storage prefixes: version 1 uses var/storage, version 2 kept the demo-inherited var/ibexa_demo_site/storage. The latter does not match the default var_dir/storage_dir, so fully loading that content throws InvalidBinaryPrefixException — breaking any consumer whose test happens to load content 54 with its image field. This aligns the prefix (dirpath, url and filepath) and adds a lib test asserting every stored image path in the fixture uses the default prefix.

The stale row dates back to the initial Added test core commit (2023-05-18) and is present on all branches. No test in ibexa/core or ibexa/test-core asserts that path.

For QA:

Nothing to test manually. TestDataTest fails on both assertions before the data change and passes after it.

Documentation:

[Steveb-p]

What's the upgrade path for existing data?