Introduce new utilities to create artificial jets for (unit) testing

This MR introduce new functions which provide a standardized way to build artificial jets. The goal is to use these artificial jets in unit test.

The changes affect only the xAODJet package (no impact on dependencies). Only the unit test in this package are affected.

Related to discussions in : https://its.cern.ch/jira/browse/ATLJETMET-1194