Deflake unit test DBCompactionTest.CompactionLimiter (#12596)

Summary:
The test has been flaky for a long time. A recent [failure](https://github.com/facebook/rocksdb/actions/runs/8820808355/job/24215219590?pr=12578) shows that there is still flush running when the assertion fails. I think this is because `WaitForFlushMemTable()` may return before the a flush schedules the next compaction.

Pull Request resolved: facebook/rocksdb#12596

Test Plan: I could not repro the failure locally: `gtest-parallel --repeat=8000 --workers=100 ./db_compaction_test --gtest_filter="*CompactionLimiter*"`

Reviewed By: ajkr

Differential Revision: D56715874

Pulled By: cbi42

fbshipit-source-id: f5f64eb30fff7e115c19beedad2dc22afa06258d

(cherry picked from commit e2ef349f56f99ae83d2ded1de23ff0684c66e1bb)