Replace ThreeVector with direct init of FourMoment in tests
This MR removes the only direct occurrences of a deprecated class HepMC::ThreeVector
in Athena.
Reasons:
- This deprecated class is absent in HepMC3 and the migration to HepMC3 would require an update of this code.
Side benefits:
- with direct initialisation of the
HepMC::FourVector
it is clear what are the values of theHepMC::FourVector
components.