trigger_comparison: output in workdir, not homedir

This matches what's done with the backgroundstats_comparison test.

@alex.codoreanu - this is the question I asked you the other day about the correct place we should be putting the output from the trigger_comparison test (which currently seems to be generating output partially relative to the current directory and partially in my home directory).

I’ve marked it Draft because I (a) wanted your thoughts and (b) think we might need to add more to finalise this one.