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

189 tests   - 34   189 ✅  - 34   8m 15s ⏱️ +41s
 38 suites  -  8     0 💤 ± 0 
 38 files    -  8     0 ❌ ± 0 

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