add_spec.rb: skip unstable spec on mingw

This sometimes crashes mspec worker like
https://ci.appveyor.com/project/ruby/ruby/build/9806/job/37tx2atojy96227m

and it doesn't generate helpful output and it seems hard to fix it soon.
As AppVeyor is too unstable by too many factors, let me skip this
at least for a short term. until AppVeyor gets stable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64933 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/threadgroup/add_spec.rb

@@ -13,6 +13,7 @@ describe "ThreadGroup#add" do
     @thread.join
   end
 
+  platform_is_not :mingw do
     it "adds the given thread to a group and returns self" do
       @thread.group.should_not == nil
 
@@ -34,3 +35,4 @@ describe "ThreadGroup#add" do
       tg1.list.include?(@thread).should == false
     end
   end
+end