Auto merge of #5185 - matklad:printf-debugging-for-life, r=alexcrichton

Add a helper method to show stdout/err in tests

So, all this time debugging Cargo was rather annoying to me, because added `println!`s tend not to show up in the test output, because they happen in the child process. So, let's add a method which just streams the subordinate process output?

@alexcrichton I wonder how you usually debug Cargo tests? Or ... are you just not making bugs to begin with? :)