Add a test for all uninitialised data handles

Discussed in ATR-21690

A DataHandle cannot be contructed from an un-initialised key. However, this might not be discovered until late in an HLT job since the CF tree is not fully explored in most events.

Added a test to the AthAlgStartVisitor to ensure that all handles have been initialised, before the event loop begins.

(Even if an uninitialised key is never used, the data-dependency remains, leading to unnecessary constraints on the scheduler)