Deflake MySQLStyleTransactionTest.TransactionStressTest in "status checked" mode (#8947)

Summary:
There is a corner case when using WriteUnprepared transactions when
`WriteUnpreparedTxn::Get` returns `Status::TryAgain` instead of
propagating the result of `GetFromBatchAndDB`. The patch adds
`PermitUncheckedError` to make the `ASSERT_STATUS_CHECKED` build pass in
this case as well.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8947

Test Plan: `make check`

Reviewed By: riversand963

Differential Revision: D31125422

Pulled By: ltamasi

fbshipit-source-id: 42de51dcfa9384e032244c2b4d3f40e9a4111194

utilities/transactions/write_unprepared_txn.cc

@@ -968,6 +968,7 @@ Status WriteUnpreparedTxn::Get(const ReadOptions& options,
              wupt_db_->ValidateSnapshot(snap_seq, backed_by_snapshot))) {
     return res;
   } else {
+    res.PermitUncheckedError();
     wupt_db_->WPRecordTick(TXN_GET_TRY_AGAIN);
     return Status::TryAgain();
   }