diff --git a/test/mixin.rb b/test/mixin.rb
index <HASH>..<HASH> 100644
--- a/test/mixin.rb
+++ b/test/mixin.rb
@@ -18,6 +18,7 @@ module MixinTest
     def teardown
       super
       Timecop.return
+      GC.start
     end
 
     module Checker
Fix test failure at in_object_space.

Depending on GC timing, mock object at test/mixin.rb are remains while
test/in_object_space.rb. But lib/fluentd/in_object_space.rb call Object#class
on all un-GCed object, including mock object.

Because unexpected method expectations are happen, in_object_space reports failure.