Exclude MergeInProgress status from errors in stress tests (#6257)

Summary:
When called on transactions, MultiGet could return a legit MergeInProgress status. The patch excludes this case from errors.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6257

Differential Revision: D19275787

Pulled By: maysamyabandeh

fbshipit-source-id: f7158229422af015947e592ae066b4273c9fb9a4

db_stress_tool/no_batched_ops_stress.cc

@@ -242,6 +242,9 @@ class NonBatchedOpsStressTest : public StressTest {
       } else if (s.IsNotFound()) {
         // not found case
         thread->stats.AddGets(1, 0);
+      } else if (s.IsMergeInProgress() && use_txn) {
+        // With txn this is sometimes expected.
+        thread->stats.AddGets(1, 1);
       } else {
         // errors case
         fprintf(stderr, "MultiGet error: %s\n", s.ToString().c_str());