Fix use of TrigDecisionTool in simple macro, and add example as a unittest - TEvent will now fire BeginEvent incidents

This MR will hopefully help preserve the knowledge of how to do simple tests with tools outside of a package-based environment.

The MR makes TEvent fire the BeginEvent incidents, which were previously being done by the AthenaEventLoopMgr but has since been disabled in athena. This is essential in the case of a simple eventloop job, which would otherwise require the scheduling of the AthIncFirerAlg in such loops.