add ability to dry-run a parallel job

[Leah Klearman]

if you're trying to troubleshoot your group_regex, you shouldn't have to run the whole job and then pick apart the results filtering by worker in order to figure out if the regex did the right thing. maybe allow for the combination of --parallel and --list ?

[Robert Collins (lifeless)]

I'm not quite sure what the UI should be, but clearly this is an important use case.

[Robert Collins (lifeless)]

Just making some notes for anyone who wants to do this:

 - need to preserve the existing command (test enumeration is needed)
 - shouldn't depend on re-invoking - we may want a command interface in future for online scheduling,so this should be explicitly handled in testr
 - dry-run is rougly equivalent to --list-tests, in that that also enumerates and doesn't run, so --list-tests --parallel could perhaps be a way forward
 - another way would be to enumerate and schedule, and output via subunit with worker tags