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

404 tests   - 1   404 ✅  - 1   12m 38s ⏱️ + 1m 51s
 39 suites ±0     0 💤 ±0 
 39 files   ±0     0 ❌ ±0 

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