Fix failing timeout test

```
Run options:
  "--ruby=./miniruby -I../ruby/lib -I. -I.ext/common  ../ruby/tool/runruby.rb --extout=.ext  -- --disable-gems"
  --excludes-dir=../ruby/test/.excludes
  --name=!/memory_leak/
  --seed=9843

[ 1/31] TestTimeout#test_timeout_in_trap_handler = 0.00 s
1) Error:
TestTimeout#test_timeout_in_trap_handler:
NoMethodError: undefined method 'kill' for nil
    /Users/luke/workspace/ruby-dev/ruby/test/test_timeout.rb:9:in 'TestTimeout#kill_timeout_thread'
    /Users/luke/workspace/ruby-dev/ruby/test/test_timeout.rb:424:in 'TestTimeout#test_timeout_in_trap_handler'

Finished tests in 2.715032s, 11.4179 tests/s, 52.3014 assertions/s.
31 tests, 142 assertions, 0 failures, 1 errors, 0 skips

ruby -v: ruby 4.0.0dev (2025-12-11T21:56:23Z fix_timeout_test 1c5eacbf9a) +PRISM [arm64-darwin24]
make: *** [yes-test-all] Error 1
```

Author: luke-gruber

test/test_timeout.rb

@@ -6,8 +6,10 @@ class TestTimeout < Test::Unit::TestCase
 
   private def kill_timeout_thread
     thread = Timeout.const_get(:State).instance.instance_variable_get(:@timeout_thread)
-    thread.kill
-    thread.join
+    if thread
+      thread.kill
+      thread.join
+    end
   end
 
   def test_public_methods