[VECGEOM-621] Resolve linking errors to Geant4 11.2 targets

Geant4 11.2 split the G4persistency target into three (G4gdml, G4geomtext and G4mctruth), but did not provide a back compatible G4persistency alias/interface target. The VecGeomTest target that links to G4persistency will thus fail with Geant4 11.2.

Update generator expression in VecGeomTest to check for existence of G4persistency target. Link to G4gdml target if this does not exist. This is forward compatible with future patches to Geant4 itself that should provide an interface/alias target for G4persistency. To suppor