runtime: test Goexit/defer iteraction.
runtime: test Goexit/defer iteraction.

Make sure Goexit runs defers.
Make sure recover() during a Goexit defer returns nil.

LGTM=dvyukov, bradfitz
R=golang-codereviews, dvyukov, bradfitz, khr
CC=golang-codereviews
https://codereview.appspot.com/140650043

bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,bryanxu/go-zh,webfd/go-zh,webfd/go-zh,bryanxu/go-zh,d0f/go-zh,webfd/go-zh,webfd/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,sanjosh/sanjos100-tipc