[5.x] Correct test namespaces to avoid PSR-4 warnings

[duncanmcclean]

This pull request fixes incorrect namespaces to avoid PSR-4 warnings from Composer.

Class Feature\Assets\SvgThumbnailTest located in ./tests/Feature/Assets/SvgThumbnailTest.php does not comply with psr-4 autoloading standard (rule: Tests\ => ./tests). Skipping.
Class Feature\Assets\ImageThumbnailTest located in ./tests/Feature/Assets/ImageThumbnailTest.php does not comply with psr-4 autoloading standard (rule: Tests\ => ./tests). Skipping.
Class Feature\Assets\PdfThumbnailTest located in ./tests/Feature/Assets/PdfThumbnailTest.php does not comply with psr-4 autoloading standard (rule: Tests\ => ./tests). Skipping.
Class Feature\Assets\DownloadAssetTest located in ./tests/Feature/Assets/DownloadAssetTest.php does not comply with psr-4 autoloading standard (rule: Tests\ => ./tests). Skipping.

Related:

• [5.x] Add auth to asset routes #13810
• [5.x] Asset auth fix #13883