Add unit tests to ttlcache to make sure cache eviction works

[haoming29]

I'm not sure if we want the ttlcache to automatically evict expired cache entry but if we do, according to the documentation here, we seem not to call go cache.Start() anywhere in our code and we don't seem to call cache.DeleteExpired() periodically which are the only two ways to make the eviction work. So I propose we add some unit tests to at least one instance of ttlcache we used to ensure the eviction work.