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

test/ruby/test_file_exhaustive.rb

@@ -514,7 +514,7 @@ class TestFileExhaustive < Test::Unit::TestCase
     assert_equal File.join(new_home, "bar", "foo"), File.expand_path("foo", "~/bar"), bug8034
 
     assert_raise(ArgumentError) { File.expand_path(".", UnknownUserHome) }
-    assert_nothing_raised(ArgumentError) { File.expand_path("/", UnknownUserHome) }
+    assert_nothing_raised(ArgumentError) { File.expand_path("#{DRIVE}/", UnknownUserHome) }
   ensure
     ENV["HOME"] = home
   end