Update the benchmark to commit t1 before starting t2

Summary:
The previous behavior was incorrect, since the second transaction
cannot see the record created in the first, because it commited later.

Reviewers: florijan, buda, dgleich

Reviewed By: dgleich

Subscribers: pullbot, matej.gradicek

Differential Revision: https://phabricator.memgraph.io/D390

tests/benchmark/mvcc.cpp

@@ -18,8 +18,8 @@ void MvccMix(benchmark::State &state) {
     auto t1 = engine.begin();
     mvcc::VersionList<Prop> version_list(*t1);
 
-    auto t2 = engine.begin();
     t1->commit();
+    auto t2 = engine.begin();
 
     state.ResumeTiming();
     version_list.update(*t2);