test: remove image_url references from test files

[zigzagdev]

Tasks:

• WorldHeritageQueryService_countEachRegionTest.php
• WorldHeritageEntityCollectionTest.php
• WorldHeritageEntityTest.php
• WorldHeritageDtoCollectionTest.php
• WorldHeritageDtoSummaryFactoryTest.php
• GetWorldHeritageByIdDtoTest.php