Remove "reference" field from JSON test files
Summary:
Instead simply prepend "Ref/" to output ROOT file name. Related with !23.
Cherry-picked 0adb52b6c0abe3bcdcf57db8698f09f72a84dae0 as it can be singled out nicely from !22 to make that MR lighter.
This is related with the general thread-safety modifications: T168
Test Plan: Output of tests is exactly the same, only reference location changed.
Reviewers: tlatham, kreps, jback
Reviewed By: jback
Tags: #evtgen
Differential Revision: https://phab.hepforge.org/D120