Add support for run-pass-valgrind tests

r? @alexcrichton 
This is somewhat worrying to me as it's one of the reasons we stopped valgrinding in the first place. We don't actually have any guarantee that any of the bots are valgrinding because they're silently ignoring bots which don't have valgrind installed.

Would it be possible to generate an error if `valgrind_path` is none? It would likely involve special logic to ignore platforms like windows and maybe android, but I would rather the entire test suite fail if it can't run valgrind.