Fix some missing values in stress test (#13039)

Summary:
When `avoid_flush_during_shutdown` is false, DB will flush the memtables if there is some unpersisted data:
79790cf2a8/db/db_impl/db_impl.cc (L505-L510)

`has_unpersisted_data_` is a flag that is only turned on for when WAL is disabled, for example:
79790cf2a8/db/db_impl/db_impl_write.cc (L525-L528)
In other cases, it just has its default false value.

So if disableWAL is false, and avoid_flush_during_shutdown is false, close won't flush memtables. Stress test is also not flush wal/sync wal. There could be missing data, while reopen in stress test doesn't tolerate missing data. To make the test simpler, this changes it to always flush/sync wal during reopen.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/13039

Reviewed By: hx235

Differential Revision: D63494695

Pulled By: jowlyzhang

fbshipit-source-id: 8f0fd9ed50a482a3955abc0882257ecc2e95926d

db_stress_tool/db_stress_test_base.cc

@@ -3674,7 +3674,7 @@ void StressTest::Reopen(ThreadState* thread) {
   // crash-recovery verification does. Therefore it always expects no data loss
   // and we should ensure no data loss in testing.
   // TODO(hx235): eliminate the FlushWAL(true /* sync */)/SyncWAL() below
-  if (!FLAGS_disable_wal && FLAGS_avoid_flush_during_shutdown) {
+  if (!FLAGS_disable_wal) {
     Status s;
     if (FLAGS_manual_wal_flush_one_in > 0) {
       s = db_->FlushWAL(/*sync=*/true);