Exclude constantly-failing test on x86_64-darwin

* https://github.com/ruby/ruby-dev-builder/actions/runs/19973218359/job/57293388626

Author: eregon

test/test_timeout.rb

@@ -299,5 +299,6 @@ def test_ractor
 
       assert_equal :ok, r
     end;
-  end if defined?(::Ractor) && RUBY_VERSION >= '4.0'
+  end if defined?(::Ractor) && RUBY_VERSION >= '4.0' && !RUBY_PLATFORM.include?('x86_64-darwin')
+  # Exclude on x86_64-darwin as it failed 4 times out of 4 tries in the CI of ruby/ruby-dev-builder
 end