updating mock. It now wraps the calls to sync and reads config from configuration file.
I believe it's better not to use the preprocessor directive, i.e. not to put any test code in the engine_tcp. I just call the mock in the test_allreduce file. It's a file purely for testing purposes, so it's fine to use the mock there.
It shouldn't be an assert because it shutdowns the process. Instead should check on the value and return some sort of error, so that we can recover.
The mock contains queues, indexed by the rank of the process. For each node, you can configure the behavior you expect (success or failure for now) when you call any of the methods (AllReduce, Broadcast, LoadCheckPoint and CheckPoint)... If you call several times AllReduce, the outputs will pop from the queue, i.e., first you can retrieve a success, then a failure and so on.
Pretty basic for now, need to tune it better
