Attempt to deflake DBFlushTest.FireOnFlushCompletedAfterCommittedResult (#9083)

Summary:
DBFlushTest.FireOnFlushCompletedAfterCommittedResult uses test sync
points to coordinate interleaving of different threads. Before this PR,
the test writes some data to memtable, triggers a manual flush, and
triggers a second manual flush after a first bg flush thread starts
executing. Though unlikely, it is possible for the second bg flush
thread to run faster than the first bg flush thread and deques flush
queue first. In this case, the original test will fail.
The fix is to wait until the first bg flush thread deques the flush
queue before triggering second manual flush.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9083

Test Plan: ./db_flush_test --gtest_filter=DBFlushTest.FireOnFlushCompletedAfterCommittedResult

Reviewed By: jay-zhuang

Differential Revision: D31951239

Pulled By: riversand963

fbshipit-source-id: f32d7cdabe6ad6808fd18e54e663936dc0a9edb4

db/db_flush_test.cc

@@ -1534,7 +1534,7 @@ TEST_F(DBFlushTest, FireOnFlushCompletedAfterCommittedResult) {
   std::shared_ptr<TestListener> listener = std::make_shared<TestListener>();
 
   SyncPoint::GetInstance()->LoadDependency(
-      {{"DBImpl::BackgroundCallFlush:start",
+      {{"DBImpl::FlushMemTableToOutputFile:AfterPickMemtables",
         "DBFlushTest::FireOnFlushCompletedAfterCommittedResult:WaitFirst"},
        {"DBImpl::FlushMemTableToOutputFile:Finish",
         "DBFlushTest::FireOnFlushCompletedAfterCommittedResult:WaitSecond"}});