Fix TruthJet writing configuration and add a unit test
During the HepMC3 + TruthJet validations for MC22, it was noticed that there was a problem with copying TruthJets to the background minbias HITS files.
This fix is required in 22.0 also, but merging it into master first as we need the change more urgently there.
@Tagging @tadej, @akraszna, @tlari, @mbandier, @gcallea, @dhayden