Move ScenarioRunner class to a separate module

For the sake of good structural code organization, we move here the central
benchmark engine class, namely the ScenarioRunner, to a separate module. We
also change the test code correspondingly and slightly rename some of the
methods left in the 'utils' module.

Blueprint benchmark-engine-refactoring

Change-Id: Id2cdad7f34c2ce067ad155c13166e256072a47e6