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

385 tests   - 8   385 ✅  - 8   8m 57s ⏱️ - 4m 24s
 24 suites  - 2     0 💤 ±0 
 24 files    - 2     0 ❌ ±0 

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