Ruby

spawn_spec.rb: add missing platform guard

this exists for context "when passed close_others: false" do, but this
seems to be missing for context "when passed close_others: true".
And this seems to hang forever on Windows since r66622.
https://ci.appveyor.com/project/ruby/ruby/builds/21277729

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66627 b2dd03c8-39d4-4d8f-98ff-823fe69b080e