test_fileutils.rb: use Dir.mktmpdir
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56209 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
test_fileutils.rb: use Dir.mktmpdir
fixed name for temporary directory.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@56209 b2dd03c8-39d4-4d8f-98ff-823fe69b080e