Ability to mark a local package as "don't test or bench"

[snoyberg]

See:

• Support deps depending on local packages #198 (comment)
• http://www.reddit.com/r/haskell/comments/3957e1/announcing_first_public_beta_of_stack/cs0frp1

[snoyberg]

I implemented this as part of #199, marking as need confirmation on that work being merged in.