Skip an unstable Ractor test for windows (ruby#14247)

https://github.com/ruby/ruby/actions/runs/16995599804/job/48185434078?pr=14242

Author: k0kubun

test/ruby/test_ractor.rb

@@ -164,7 +164,7 @@ def test_require_non_string
 
   # [Bug #21398]
   def test_port_receive_dnt_with_port_send
-    omit 'unstable on macos-14' if RUBY_PLATFORM =~ /darwin/
+    omit 'unstable on windows and macos-14' if RUBY_PLATFORM =~ /mswin|darwin/
     assert_ractor(<<~'RUBY', timeout: 90)
       THREADS = 10
       JOBS_PER_THREAD = 50