ruby: Fix forking with multiple Ractors

[wks]

We fixed a racing bug related to forking while there are multiple Ractors.

[wks]

One assertion failed

2025-11-11T07:01:06.8823129Z   1) Failure:
2025-11-11T07:01:06.8823563Z TestIO#test_set_lineno_gets [/home/runner/work/mmtk-ruby/mmtk-ruby/git/ruby/test/ruby/test_io.rb:1967]:
2025-11-11T07:01:06.8824024Z <1001> expected but was
2025-11-11T07:01:06.8824212Z <39>.

The test case tests the $. global variable which records the line number of the last IO#gets invocation. If tests are running in multiple threads, there is a chance that another test case performs another IO#gets right between the current thread runs IO#gets and evaluates $.. If that happens, it will overwrite the $. and make the test case fail.

I'll restart the tests.

[wks]

TestResolvDNS#test_multiple_servers_with_timeout_and_truncated_tcp_fallback timed out. It looks like a network failure. I'll rerun the test again.