fix sporadic thread_safety test failure

For this test, the spawned thread was ending after the main block was running

When the main block quit, it undefined the method on the class.
The threaded class kept calling the method (now undefined) and threw an exception
because the method was no longer defined.

solution is to wait for the thread to finish before exiting and undefining the method

spec/core_ext/module/cache_with_timeout_spec.rb

@@ -163,7 +163,7 @@
       #   clears the current value, the other thread could get nil.
       test_class.cache_with_timeout(:thread_safety) { 2 }
 
-      Thread.new do
+      t = Thread.new do
         10000.times do
           test_class.thread_safety(true)
         end
@@ -172,6 +172,7 @@
       10000.times do
         expect(test_class.thread_safety).to eq(2)
       end
+      t.join
     end
   end
 end