RemoteTestSet to preserve testset metadata across processes

[christiangnrd]

Is this too cursed? The interface would be that custom runtest (or execute after #51) functions would use @remote_testset instead of @testset.

If we go this route I'll add tests.

Close #32

[vchuravy]

@christiangnrd I didn't see this PR until I had already written #55
I have a slight preference over my variant since we don't need a new macro.

I am still missing the "failed at" print, but that should be a relatively straightforward scan over the results.

[christiangnrd]

Closing in favour of #55