For avoiding the following error on Ruby-3.1 on Windows:
```
2022-02-18T01:36:00.3691879Z ===============================================================================
2022-02-18T01:36:00.3692453Z Failure: test: send tags in str (utf-8 strings)(ForwardOutputTest):
2022-02-18T01:36:00.3692799Z On subject ,
2022-02-18T01:36:00.3693302Z unexpected method invocation:
2022-02-18T01:36:00.3693595Z process_wait(2088, 0)
2022-02-18T01:36:00.3702794Z expected invocations:
2022-02-18T01:36:00.3703495Z C:/hostedtoolcache/windows/Ruby/3.1.0/x64/lib/ruby/gems/3.1.0/gems/serverengine-2.2.5/lib/serverengine/socket_manager.rb:79:in ``'
2022-02-18T01:36:00.3704338Z C:/hostedtoolcache/windows/Ruby/3.1.0/x64/lib/ruby/gems/3.1.0/gems/serverengine-2.2.5/lib/serverengine/socket_manager.rb:79:in `block in generate_path'
2022-02-18T01:36:00.3705025Z C:/hostedtoolcache/windows/Ruby/3.1.0/x64/lib/ruby/gems/3.1.0/gems/serverengine-2.2.5/lib/serverengine/socket_manager.rb:78:in `each'
2022-02-18T01:36:00.3705695Z C:/hostedtoolcache/windows/Ruby/3.1.0/x64/lib/ruby/gems/3.1.0/gems/serverengine-2.2.5/lib/serverengine/socket_manager.rb:78:in `generate_path'
2022-02-18T01:36:00.3706156Z D:/a/fluentd/fluentd/lib/fluent/test/driver/base.rb:105:in `instance_start'
2022-02-18T01:36:00.3706550Z D:/a/fluentd/fluentd/lib/fluent/test/driver/base.rb:77:in `run'
2022-02-18T01:36:00.3706945Z D:/a/fluentd/fluentd/lib/fluent/test/driver/base_owner.rb:130:in `run'
2022-02-18T01:36:00.3707422Z D:/a/fluentd/fluentd/test/plugin/test_out_forward.rb:434:in `block in <class:ForwardOutputTest>'
2022-02-18T01:36:00.3707780Z 431: ]
2022-02-18T01:36:00.3708021Z 432:
2022-02-18T01:36:00.3708366Z 433: stub(d.instance.ack_handler).read_ack_from_sock(anything).never
2022-02-18T01:36:00.3708758Z => 434: target_input_driver.run(expect_records: 2) do
2022-02-18T01:36:00.3709053Z 435: d.run do
2022-02-18T01:36:00.3709364Z 436: emit_events.each do |tag, t, record|
2022-02-18T01:36:00.3709681Z 437: d.feed(tag, t, record)
2022-02-18T01:36:00.3709997Z ===============================================================================
```
Signed-off-by: Takuro Ashie <ashie@clear-code.com>