Wait for #try_flush to finish

and wait until next interval
https://github.com/fluent/fluentd/blob/02f34e74b2209281ac56974722be1d449cf4dbc0/lib/fluent/plugin/output.rb#L1462

Signed-off-by: Yuta Iwama <ganmacs@gmail.com>

test/plugin/test_output_as_buffered_retries.rb

@@ -895,6 +895,10 @@ def get_log_time(msg, logs)
 
       @i.flush_thread_wakeup
       waiting(4){ Thread.pass until @i.write_count > 0 }
+      waiting(4) do
+        state = @i.instance_variable_get(:@output_flush_threads).first
+        state.thread.status == 'sleep'
+      end
 
       assert(@i.write_count > 0)
       assert(@i.num_errors > 0)
@@ -903,6 +907,10 @@ def get_log_time(msg, logs)
       Timecop.freeze( now )
       @i.flush_thread_wakeup
       waiting(4){ Thread.pass until @i.write_count > 1 }
+      waiting(4) do
+        state = @i.instance_variable_get(:@output_flush_threads).first
+        state.thread.status == 'sleep'
+      end
 
       assert(@i.write_count > 1)
       assert(@i.num_errors > 1)