Fix unit test failure ppc64le in travis (#7752)

Summary:
Added a fix for the failure of
DBTest2.PartitionedIndexUserToInternalKey on ppc64le in travis
Closes https://github.com/facebook/rocksdb/issues/7746

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7752

Test Plan:
Ran travis job multiple times and it passed. Will keep
watching the travis job after this patch.

Reviewed By: pdillinger

Differential Revision: D25373130

Pulled By: akankshamahajan15

fbshipit-source-id: fa0e3f85f75b687415044a506e42cc38ead87975

db/db_test2.cc

@@ -174,7 +174,9 @@ TEST_F(DBTest2, PartitionedIndexUserToInternalKey) {
     ASSERT_OK(Put("keykey_" + std::to_string(j), value));
     snapshots.push_back(db_->GetSnapshot());
   }
-  Flush();
+  ASSERT_OK(Flush());
+  dbfull()->TEST_WaitForFlushMemTable();
+
   for (auto s : snapshots) {
     db_->ReleaseSnapshot(s);
   }