Fix random timeout in test_thread_join_during_finalizers on Windows (ruby#16650)

Reduce the number of spawned processes from 50 to 20 and increase the
timeout from 30s to 60s. Process creation is expensive on Windows, and
the combination caused the test to exceed the 30s limit on Windows CI.

Author: k0kubun

test/ruby/test_thread.rb

@@ -1667,7 +1667,7 @@ def test_mn_threads_sub_millisecond_sleep
 
   # [Bug #21926]
   def test_thread_join_during_finalizers
-    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}", timeout: 30)
+    assert_separately([], "#{<<~"begin;"}\n#{<<~'end;'}", timeout: 60)
     begin;
       require 'open3'
 
@@ -1690,7 +1690,7 @@ def self.make_finalizer(stdin, stdout, stderr, wait_thread)
         end
       end
 
-      50.times { ProcessWrapper.new }
+      20.times { ProcessWrapper.new }
       GC.stress = true
       1000.times { Object.new }
     end;