Fix DBFlushTest::FireOnFlushCompletedAfterCommittedResult hang (#6018)

Summary:
The test would fire two flushes to let them run in parallel. Previously it wait for the first job to be scheduled before firing the second. It is possible the job is not started before the second job being scheduled, making the two job combine into one. Change to wait for the first job being started.

Fixes https://github.com/facebook/rocksdb/issues/6017
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6018

Test Plan:
```
while ./db_flush_test --gtest_filter=*FireOnFlushCompletedAfterCommittedResult*; do :; done
```
and let it run for a while.

Signed-off-by: Yi Wu <yiwu@pingcap.com>

Differential Revision: D18405576

Pulled By: riversand963

fbshipit-source-id: 6ebb6262e033d5dc2ef81cb3eb410b314f2de4c9

db/db_flush_test.cc

@@ -370,7 +370,7 @@ TEST_F(DBFlushTest, FireOnFlushCompletedAfterCommittedResult) {
   std::shared_ptr<TestListener> listener = std::make_shared<TestListener>();
 
   SyncPoint::GetInstance()->LoadDependency(
-      {{"DBImpl::FlushMemTable:AfterScheduleFlush",
+      {{"DBImpl::BackgroundCallFlush:start",
         "DBFlushTest::FireOnFlushCompletedAfterCommittedResult:WaitFirst"},
        {"DBImpl::FlushMemTableToOutputFile:Finish",
         "DBFlushTest::FireOnFlushCompletedAfterCommittedResult:WaitSecond"}});
@@ -401,7 +401,7 @@ TEST_F(DBFlushTest, FireOnFlushCompletedAfterCommittedResult) {
     // flush_opts.wait = true
     ASSERT_OK(db_->Flush(FlushOptions()));
   });
-  // Wait for first flush scheduled.
+  // Wait for first flush started.
   TEST_SYNC_POINT(
       "DBFlushTest::FireOnFlushCompletedAfterCommittedResult:WaitFirst");
   // The second flush will exit early without commit its result. The work