Attempt to deflake ListenerTest.MultiCF (#9084)

Summary:
EventListenerTest.MultiCF uses TestFlushListener which has members
flushed_dbs_ and flushed_column_family_names_ that are not protected by
locks. This implicitly indicates that we need to ensure the methods
accessing these data structures in a single threaded way. In other
tests, e.g. MultiDBMultiListeners, we use TEST_WaitForFlushMemtable() to
wait until all memtables of a given column family are flushed, hence no
pending flush threads will concurrently call OnFlushCompleted() and
cause data race for flushed_dbs_. To fix a test failure, we should do
the same for MultiCF.

Example data race stack traces reported by TSAN
```
Read of size 8 at 0x7b6000002840 by main thread:
    #0 std::vector<rocksdb::DB*, std::allocator<rocksdb::DB*> >::size() const /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/stl_vector.h:655:40
    https://github.com/facebook/rocksdb/issues/1 rocksdb::EventListenerTest_MultiCF_Test::TestBody() /home/circleci/project/db/listener_test.cc:380:7

Previous write of size 8 at 0x7b6000002840 by thread T2:
    #0 void std::vector<rocksdb::DB*, std::allocator<rocksdb::DB*> >::_M_emplace_back_aux<rocksdb::DB* const&>(rocksdb::DB* const&) /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/vector.tcc:442:26
    https://github.com/facebook/rocksdb/issues/1 std::vector<rocksdb::DB*, std::allocator<rocksdb::DB*> >::push_back(rocksdb::DB* const&) /usr/bin/../lib/gcc/x86_64-linux-gnu/5.4.0/../../../../include/c++/5.4.0/bits/stl_vector.h:923:4
    https://github.com/facebook/rocksdb/issues/2 rocksdb::TestFlushListener::OnFlushCompleted(rocksdb::DB*, rocksdb::FlushJobInfo const&) /home/circleci/project/db/listener_test.cc:255:18
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9084

Test Plan: ./listener_test --gtest_filter=EventListenerTest.MultiCF

Reviewed By: jay-zhuang

Differential Revision: D31952259

Pulled By: riversand963

fbshipit-source-id: 94a7f29e4e9466ead42418944eb2247fc32bd499

db/listener_test.cc

@@ -377,6 +377,7 @@ TEST_F(EventListenerTest, MultiCF) {
     ASSERT_OK(Put(7, "popovich", std::string(90000, 'p')));
     for (int i = 1; i < 8; ++i) {
       ASSERT_OK(Flush(i));
+      ASSERT_OK(dbfull()->TEST_WaitForFlushMemTable());
       ASSERT_EQ(listener->flushed_dbs_.size(), i);
       ASSERT_EQ(listener->flushed_column_family_names_.size(), i);
     }