test_supervisor: fix flaky tests for Win32 event (#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.


https://github.com/fluent/fluentd/blob/6d26f8de536601d9c192476f00cd22925a8882ad/test/test_supervisor.rb#L417-L421

**Docs Changes**:
N/A

**Release Note**: 
N/A

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

test/test_supervisor.rb

@@ -443,6 +443,7 @@ def server.config
       event = Win32::Event.open("TestFluentdEvent")
       event.set
       event.close
+      sleep 1.0 # Wait for dumping
     ensure
       server.stop_windows_event_thread
     end
@@ -469,6 +470,7 @@ def server.config
       event = Win32::Event.open("TestFluentdEvent_USR1")
       event.set
       event.close
+      sleep 1.0 # Wait for dumping
     ensure
       server.stop_windows_event_thread
     end