It's timed out on CI like this
https://gist.github.com/ko1/33df53d78fbec9e4156c017eeae023bb.
I'm not sure how long it would take (it finishes immediately on my machine),
so I chose 600s because r64271 says it took 635s on mswinci. (probably
it's improved by the commit, though.)
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e