CI: Fails "test: using child processes by round robin[with sections](ExecFilterOutputTest)" test

[Watson1978]

Describe the bug

Sometimes, it fails with following error message:

1) Failure: test: using child processes by round robin[with sections](ExecFilterOutputTest)
D:/a/fluentd/fluentd/test/plugin/test_out_exec_filter.rb:514:in `block in <class:ExecFilterOutputTest>'
     511:     assert_equal "2011-01-02 13:14:15\ttest\t3\n", d.formatted[3]
     512: 
     513:     events = d.events
  => 514:     assert_equal 4, events.length
     515: 
     516:     pid_list = []
     517:     events.each do |event|
<4> expected but was
<3>
Error: <4> expected but was
<3>.

To Reproduce

Run CI.

Expected behavior

No error

Your Environment

- Fluentd version:
- Package version:
- Operating system:
- Kernel version:

Your Configuration

none

Your Error Log

See above

Additional context

No response

[Watson1978]

It seems that the command is being executed asynchronously with d.run.

fluentd/test/plugin/test_out_exec_filter.rb

Lines 503 to 506 in 6d4e0d0

	d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: true, shutdown: false){ d.feed(time, {"k1" => 0}) }
	d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 1}) }
	d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 2}) }
	d.run(default_tag: 'test', expect_emits: 1, timeout: 10, start: false, shutdown: false){ d.feed(time, {"k1" => 3}) }

When I checked the value of d.emit_count when it failed, it was d.emit_count == 3.

Looks for me that it is better to insert a short waiting time.