Skip a new spec for Dir#chdir on Windows when original directory is removed

The following error is raised:

```
Dir#chdir does not raise an Errno::ENOENT if the original directory no longer exists ERROR
Errno::ENOENT: No such file or directory @ dir_chdir0 - D:/a/spec/spec/rubyspec_temp/5588_0/27-testdir1
D:/a/spec/spec/core/dir/chdir_spec.rb:192:in `chdir'
D:/a/spec/spec/core/dir/chdir_spec.rb:192:in `block (4 levels) in <top (required)>'
D:/a/spec/spec/core/dir/chdir_spec.rb:191:in `chdir'
D:/a/spec/spec/core/dir/chdir_spec.rb:191:in `block (3 levels) in <top (required)>'
D:/a/spec/spec/core/dir/chdir_spec.rb:129:in `block in <top (required)>'
D:/a/spec/spec/core/dir/chdir_spec.rb:128:in `<top (required)>'
```

Author: andrykonchin

core/dir/chdir_spec.rb

@@ -177,27 +177,29 @@ def to_str;  DirSpecs.mock_dir; end
       dir.close
     end
 
-    it "does not raise an Errno::ENOENT if the original directory no longer exists" do
-      dir_name1 = tmp('testdir1')
-      dir_name2 = tmp('testdir2')
-      Dir.should_not.exist?(dir_name1)
-      Dir.should_not.exist?(dir_name2)
-      Dir.mkdir dir_name1
-      Dir.mkdir dir_name2
-
-      dir2 = Dir.new(dir_name2)
-
-      begin
-        Dir.chdir(dir_name1) do
-          dir2.chdir { Dir.unlink dir_name1 }
+    platform_is_not :windows do
+      it "does not raise an Errno::ENOENT if the original directory no longer exists" do
+        dir_name1 = tmp('testdir1')
+        dir_name2 = tmp('testdir2')
+        Dir.should_not.exist?(dir_name1)
+        Dir.should_not.exist?(dir_name2)
+        Dir.mkdir dir_name1
+        Dir.mkdir dir_name2
+
+        dir2 = Dir.new(dir_name2)
+
+        begin
+          Dir.chdir(dir_name1) do
+            dir2.chdir { Dir.unlink dir_name1 }
+          end
+          Dir.pwd.should == @original
+        ensure
+          Dir.unlink dir_name1 if Dir.exist?(dir_name1)
+          Dir.unlink dir_name2 if Dir.exist?(dir_name2)
         end
-        Dir.pwd.should == @original
       ensure
-        Dir.unlink dir_name1 if Dir.exist?(dir_name1)
-        Dir.unlink dir_name2 if Dir.exist?(dir_name2)
+        dir2.close
       end
-    ensure
-      dir2.close
     end
 
     it "always returns to the original directory when given a block" do