Summary:
as reported in https://github.com/facebook/rocksdb/issues/11476, RocksDB currently does not execute compactions in two subcompactions even when they qualify. This PR fixes this issue.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11501

Test Plan:
* Add a new unit test.
* Run crash test with max_subcompactions=2: `python3 tools/db_crashtest.py blackbox --simple  --subcompactions=2 --target_file_size_base=1048576 --compaction_style=0`
  * saw logs showing compactions being executed as 2 subcompactions
```
2023/06/01-17:28:44.028470 3147486 (Original Log Time 2023/06/01-17:28:44.025972) EVENT_LOG_v1 {"time_micros": 1685665724025939, "job": 6, "event": "compaction_finished", "compaction_time_micros": 34539, "compaction_time_cpu_micros": 26404, "output_level": 6, "num_output_files": 2, "total_output_size": 1109796, "num_input_records": 13188, "num_output_records": 13021, "num_subcompactions": 2, "output_compression": "NoCompression", "num_single_delete_mismatches": 0, "num_single_delete_fallthrough": 0, "lsm_state": [0, 0, 0, 0, 0, 0, 13]}
```

Reviewed By: ajkr

Differential Revision: D46411497

Pulled By: cbi42

fbshipit-source-id: 3ebfc02e19f78f782e114a9546dc3d481d496258