Backport(v1.16): test_supervisor: fix flaky tests for Win32 event (#5174)

[Watson1978]

Backport #5174

Which issue(s) this PR fixes:
This patch will fix following failures on Windows with old Ruby (3.0, 2.7).

1) Failure: test_supervisor_event_handler(SupervisorTest): <false> is not true.
D:/a/fluentd/fluentd/test/test_supervisor.rb:388:in `test_supervisor_event_handler'
     385: 
     386:     debug_msg = '[debug]: Got Win32 event "TestFluentdEvent_USR1"'
     387:     logs = $log.out.logs
  => 388:     assert{ logs.any?{|log| log.include?(debug_msg) } }
     389:   ensure
     390:     $log.out.reset if $log&.out&.respond_to?(:reset)
     391:   end

2) Failure: test_windows_shutdown_event(SupervisorTest): <false> is not true.
D:/a/fluentd/fluentd/test/test_supervisor.rb:362:in `test_windows_shutdown_event'
     359: 
     360:     debug_msg = '[debug]: Got Win32 event "TestFluentdEvent"'
     361:     logs = $log.out.logs
  => 362:     assert{ logs.any?{|log| log.include?(debug_msg) } }
     363:   ensure
     364:     $log.out.reset if $log&.out&.respond_to?(:reset)
     365:   end

https://github.com/fluent/fluentd/actions/runs/19950323545/job/57208741261

What this PR does / why we need it:
It seems we need to wait a bit to reliably capture events as logs.

Similar to following codes, this also includes sleep.

fluentd/test/test_supervisor.rb

Lines 417 to 421 in 6d26f8d

	sleep 0.1 # Wait for starting windows event thread
	event = Win32::Event.open("TestFluentdEvent_CONT")
	event.set
	event.close
	sleep 1.0 # Wait for dumping

Docs Changes:
N/A

Release Note:
N/A

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:

Docs Changes:

Release Note:

[kenhys]

LGTM.

wait CI to pass.