spec: add comments to re-enable specs for MinGW [ci skip]

r64918 and r64988 should not be kept forever.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64992 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/core/thread/terminate_spec.rb

@@ -3,6 +3,8 @@ require_relative 'fixtures/classes'
 require_relative 'shared/exit'
 
 describe "Thread#terminate" do
+  # This spec randomly kills mspec worker like: https://ci.appveyor.com/project/ruby/ruby/builds/19390874/job/wv1bsm8skd4e1pxl
+  # TODO: Investigate the cause or at least print helpful logs, and remove this `platform_is_not` guard.
   platform_is_not :mingw do
     it_behaves_like :thread_exit, :terminate
   end

spec/ruby/core/threadgroup/add_spec.rb

@@ -13,6 +13,8 @@ describe "ThreadGroup#add" do
     @thread.join
   end
 
+  # This spec randomly kills mspec worker like: https://ci.appveyor.com/project/ruby/ruby/build/9806/job/37tx2atojy96227m
+  # TODO: Investigate the cause or at least print helpful logs, and remove this `platform_is_not` guard.
   platform_is_not :mingw do
     it "adds the given thread to a group and returns self" do
       @thread.group.should_not == nil