Skip Windows tests in non-Windows environment.

test/test_path_sanitization.rb

@@ -29,6 +29,7 @@ class TestPathSanitization < JekyllUnitTest
                  Jekyll.sanitized_path(source_dir, "f./../../../../../../files/hi.txt")
   end
 
+  if Jekyll::Utils::Platforms.really_windows?
     context "on Windows with absolute path" do
       setup do
         @base_path = "D:/demo"
@@ -54,6 +55,7 @@ class TestPathSanitization < JekyllUnitTest
                      Jekyll.sanitized_path(@base_path, @file_path)
       end
     end
+  end
 
   should "not strip base path if file path has matching prefix" do
     assert_equal "/site/sitemap.xml",