fluentd: Some unit tests are broken on Ruby 3.0

Check CONTRIBUTING guideline first and here is the list to help us investigate the problem.

Describe the bug

Drone CI is continuously broken for two months. See https://cloud.drone.io/fluent/fluentd

To Reproduce

Follow steps in .drone.yml

See https://github.com/fluent/fluentd/blob/master/.drone.yml#L26-L35

Expected behavior

There are 3 test case failures.

===============================================================================
Failure: test: it does not create accessor methods for parameters(Fluent::Config::TestConfigurable:::skip_accessor option)
/drone/src/test/config/test_configurable.rb:1456:in `block (2 levels) in <class:TestConfigurable>'
     1453:         @example = ConfigurableSpec::ExampleWithSkipAccessor.new
     1454:         @example.configure(config_element('ROOT'))
     1455:         assert_equal 'example7', @example.instance_variable_get(:@name)
  => 1456:         assert_raise NoMethodError.new("undefined method `name' for #{@example}") do
     1457:           @example.name
     1458:         end
     1459:       end

===============================================================================
Failure: test: when out_forward has @id(ForwardOutputTest):
  Exception raised:
  ArgumentError(<wrong number of arguments (given 2, expected 1)>)
  /drone/src/lib/fluent/plugin.rb:120:in `new_sd'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/method_dispatches/method_dispatch.rb:30:in `call_original_method'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/method_dispatches/method_dispatch.rb:41:in `call_implementation'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/method_dispatches/method_dispatch.rb:16:in `call'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/injections/double_injection.rb:183:in `dispatch_method'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/injections/double_injection.rb:40:in `dispatch_method'
  /drone/src/vendor/bundle/ruby/3.0.0/gems/rr-1.2.1/lib/rr/injections/double_injection.rb:148:in `new_sd'
  /drone/src/lib/fluent/plugin_helper/service_discovery/manager.rb:37:in `block in configure'

===============================================================================
Failure: test: Do not create connection per send_data(ForwardOutputTest::keepalive):
  socket_create_tcp("127.0.0.1", 38557, anything)
  Called 2 times.
  Expected 1 times.
/drone/src/test/plugin/test_out_forward.rb:1188:in `block (2 levels) in <class:ForwardOutputTest>'
     1185:
     1186:       begin
     1187:         chunk = Fluent::Plugin::Buffer::MemoryChunk.new(Fluent::Plugin::Buffer::Metadata.new(nil, nil, nil))
  => 1188:         mock.proxy(d.instance).socket_create_tcp(TARGET_HOST, TARGET_PORT, anything) { |sock| mock(sock).close.once; sock }.once
     1189:
     1190:         target_input_driver.run(timeout: 15) do
     1191:           d.run(shutdown: false) do
===============================================================================

Your Environment

• Fluentd or td-agent version: fluentd --version or td-agent --version
• Operating system: cat /etc/os-release
• Kernel version: uname -r

$ ruby -v
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [aarch64-linux]
$ bundle exec gem list

*** LOCAL GEMS ***

async (1.28.9)
async-http (0.54.1)
async-io (1.30.2)
async-pool (0.3.5)
bundler (2.2.11)
concurrent-ruby (1.1.8)
console (1.10.1)
cool.io (1.7.1)
docile (1.3.5)
fiber-local (1.0.0)
flexmock (2.3.6)
fluentd (1.12.1)
http_parser.rb (0.6.0)
msgpack (1.4.2)
nio4r (2.5.5)
oj (3.11.2)
parallel (1.20.1)
parallel_tests (0.15.4)
power_assert (2.0.0)
protocol-hpack (1.4.2)
protocol-http (0.21.0)
protocol-http1 (0.13.2)
protocol-http2 (0.14.2)
rake (13.0.3)
rr (1.2.1)
serverengine (2.2.3)
sigdump (0.2.4)
simplecov (0.21.2)
simplecov-html (0.12.3)
simplecov_json_formatter (0.1.2)
strptime (0.2.5)
test-unit (3.4.0)
test-unit-rr (1.0.5)
timecop (0.9.4)
timers (4.3.3)
tzinfo (2.0.4)
tzinfo-data (1.2021.1)
yajl-ruby (1.4.1)

Your Configuration

N/A

Your Error Log

I’ve attached logs

Additional context

N/A arm64v8-test.log