skips thread safety test

```
more_core_extensions/spec/core_ext/module/cache_with_timeout_spec.rb:168:in
`block (5 levels) in <top (required)>': undefined method `thread_safety' for

          test_class.thread_safety(true)
                    ^^^^^^^^^^^^^^
	from more_core_extensions/spec/core_ext/module/cache_with_timeout_spec.rb:167:in
	    `times'
	from more_core_extensions/spec/core_ext/module/cache_with_timeout_spec.rb:167:in
	    `block (4 levels) in <top (required)>'
```

spec/core_ext/module/cache_with_timeout_spec.rb

@@ -162,6 +162,7 @@
       #   and one tries to force reload, there is a small window where as it
       #   clears the current value, the other thread could get nil.
       test_class.cache_with_timeout(:thread_safety) { 2 }
+      skip "thread safety not supported" unless test_class.respond_to?(:thread_safety)
 
       Thread.new do
         10000.times do