Add E2E test for WorkloadState and NodeState

[jafermarq]

The E2E test is designed as follows:

• Clients append to their state the current timestamp the moment they do fit() and evaluate(). This results in the client's state to have a collection of monotonically increasing timestamps.
• The server communicates to the client their timestamps series through the metrics return argument
• The strategy checks for timestamp monotonicity after aggregation.

If test pass it means that the mechanism of injecting the WorkloadState into the client objects after being retrieved from the NodeState is working.