HepMcParticleLink unit test fixes

This merge request updates the HepMcParticleLink unit tests, so that they cover all use-cases found (so far) in current production jobs. It also updates the HepMcParticleLink class and the HepMcParticleLinkCnv_p1, HepMcParticleLinkCnv_p2, SiHitCollectionCnv_p3 and TRT_HitCollectionCnv_p4 classes so that these unit tests pass. It should also fix the problem seen in ATLASSIM-4539, which occurred when there were no HepMcParticleLinks to the hard-scatter GenEvent. (The problem appears to be that in pre-mixing jobs the McEventCollection cannot be found during T->P conversion of the SDOs for some reason.)