test_output_as_buffered_retries: fix flaky test

[Watson1978]

Which issue(s) this PR fixes:
Fixes #4873

What this PR does / why we need it:
Before invoke @num_errors_metrics.inc, the does retries correctly when #try_write fails test will be failed if thread will be switched at

fluentd/lib/fluent/plugin/output.rb

Lines 1313 to 1315 in f34a253

	def update_retry_state(chunk_id, using_secondary, error = nil)
	@retry_mutex.synchronize do
	@num_errors_metrics.inc

It is easy to reproduce by adding sleep just before @num_errors_metrics.inc, like:

diff --git a/lib/fluent/plugin/output.rb b/lib/fluent/plugin/output.rb
index a987937b..d97d8ec6 100644
--- a/lib/fluent/plugin/output.rb
+++ b/lib/fluent/plugin/output.rb
@@ -1312,6 +1312,7 @@ def check_slow_flush(start)
 
       def update_retry_state(chunk_id, using_secondary, error = nil)
         @retry_mutex.synchronize do
+          sleep 0.1
           @num_errors_metrics.inc
           chunk_id_hex = dump_unique_id_hex(chunk_id)
 

This patch will wait until @num_errors_metrics.inc is invoked.

Docs Changes:

Release Note:

[Watson1978]

Thanks!