[OPIK-8181] [BE] refactor: centralise dataset item test assertions in DatasetItemAssertions - #8095

296 tests  +15   296 ✅ +18   9m 56s ⏱️ +59s
 41 suites + 3     0 💤  -  3 
 41 files   + 3     0 ❌ ± 0 

Results for commit d863779. ± Comparison against earlier commit c79b120.