test_buf_file_single: ensure to shutdown plugin instances

Signed-off-by: Shizuo Fujita <fujita@clear-code.com>

Author: Watson1978

test/plugin/test_buf_file_single.rb

@@ -790,6 +790,12 @@ def create_driver(conf = TAG_CONF, klass = FluentPluginFileSingleBufferTest::Dum
         @p.close unless @p.closed?
         @p.terminate unless @p.terminated?
       end
+      if @d
+        @d.stop unless @d.stopped?
+        @d.before_shutdown unless @d.before_shutdown?
+        @d.shutdown unless @d.shutdown?
+        @d.after_shutdown unless @d.after_shutdown?
+      end
     end
 
     test 'worker(id=0) #resume returns staged/queued chunks with metadata, not only in worker dir, including the directory specified by path' do