ModuleResult find_by_module_and_processing

Fixed behaviour for MetricCollector:
  Now a ModuleResult may have MetricCollector heterogeneous MetricResults

Signed off by: Alessandro Palmeira <alessandro.palmeira@gmail.com>
Refactored runner_mock_helper ModuleResult#find_by

Signed off by: Alessandro Palmeira <alessandro.palmeira@gmail.com>
ModuleResult find_by_module_and_processing unit tests

Signed off by: Alessandro Palmeira <alessandro.palmeira@gmail.com>