test_io.rb: use assert_separately
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41417 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_io.rb: use assert_separately
assert_separately instead of separated fork and assert.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41417 b2dd03c8-39d4-4d8f-98ff-823fe69b080e