assertion doesn't seems to be checking the unicode string on command
line, but seems to be checking how to treat the unicode string from
stdin. so, should escape '\' before 'u'. this fixes a test failure
on Windows.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e