test/eas: add test under CPU hotplug stress

Created by: qperret

The CPU hotplug procedure changes many data structures used by the scheduler and interferes with the task placement, hence resulting in all sorts of bugs. This PR enables testing under CPU hotplug stress as an attemp to trigger those bugs. CPUs are plugged in and off in a random (but repeatable) way while a workload is running. For now, the test is considered passing if the workload simply completes its work and if the system doesn't halt.

One thing worth discussing about the current implementation: the hotplug stress starts at the very beginning and stops at the very end. It does not stop nor restart between iterations of the test, which means that iteration n and n+1 are not assessed over the same hotplug sequence. Not sure if this is a problem or a feature ...

One last thing, I'm not (yet) familiar with the all the test APIs so all feedbacks are more than welcome :)