Check for multiple instances of TestHepMC (and TestLHE?)

In general, the transform will create an instance of TestHepMC (and in the future also TestLHE) and run some checks as part of the job. For some setups the default thresholds used in these packages may be too strict and occasionally we get JOs that try to loosen them a bit, which is usually fine.

We recently had a case (!1066 (merged)) where a fresh instance of TestHepMC was created, and the threshold were tweaked on the new instance but not the one that the transform had already created, which was then causing issues down the line.

Could we catch this sort of thing in the CI? I imagine it would just be a case of checking for a line like

genSeq += TestHepMC()

and throwing an error?