test_workspace.rb: skip test failing on windows

> Note that all files are always readable
> https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/chmod-wchmod
It seems that we can't make a file unreadable with `File.chmod` on
Windows.

When file can't be read, File::EACCES is raised on Windows too. So
r60900 should work anyway, but I don't know how to let it happen by
Ruby code. I tried to open file before reading it, but I couldn't
reproduce File::EACCES too.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@60901 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/irb/test_workspace.rb

@@ -32,16 +32,18 @@ module TestIRB
       end
     end
 
-    def test_code_around_binding_with_existing_unreadable_file
-      Tempfile.create do |f|
-        code = "IRB::WorkSpace.new(binding)\n"
-        f.print(code)
-        f.close
+    unless /mswin|mingw/ =~ RUBY_PLATFORM
+      def test_code_around_binding_with_existing_unreadable_file
+        Tempfile.create do |f|
+          code = "IRB::WorkSpace.new(binding)\n"
+          f.print(code)
+          f.close
 
-        File.chmod(0, f.path)
+          File.chmod(0, f.path)
 
-        workspace = eval(code, binding, f.path)
-        assert_equal(nil, workspace.code_around_binding)
+          workspace = eval(code, binding, f.path)
+          assert_equal(nil, workspace.code_around_binding)
+        end
       end
     end