Summary:
Fix a use-after-move issue in block.cc and added some unit tests.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11505

Test Plan:
```
make all check
./block_test
```

Reviewed By: ltamasi

Differential Revision: D46506188

Pulled By: jowlyzhang

fbshipit-source-id: 316ed8ddd221c00b2bce2cf9fd47eea686cd74a5