test_file_exhaustive.rb: fix abolute root

* test/ruby/test_file_exhaustive.rb (test_expand_path_home_dir_string):
  fix abolute root path for dosish platforms.  [ruby-core:53354]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39740 b2dd03c8-39d4-4d8f-98ff-823fe69b080e