Skip an unstable Ractor test for macOS (ruby#14231)

https://github.com/ruby/ruby/actions/runs/16977094733/job/48128667252?pr=14229

Author: k0kubun

test/ruby/test_ractor.rb

@@ -164,6 +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/
     assert_ractor(<<~'RUBY', timeout: 30)
       THREADS = 10
       JOBS_PER_THREAD = 50