Backport(v1.16): test_supervisor: use expected temporary directory for tests (#5102)

[Watson1978]

Which issue(s) this PR fixes:

• Backport test_supervisor: use expected temporary directory for tests #5102

What this PR does / why we need it:
test_supervisor.rb uses a constant named TMP_DIR. However, the constant is not defined in this file, it is defined in another file.

fluentd/test/config/test_dsl.rb

Line 5 in 5a87509

TMP_DIR = File.dirname(__FILE__) + "/tmp/config_dsl#{ENV['TEST_ENV_NUMBER']}"

This PR will use expected temporary directory for tests.

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:

[daipom]

Need to check broken CI

[ashie]

Need to check broken CI

I believe these errors aren't concerned with this change.

There are 2 patterns in these errors:

https://github.com/fluent/fluentd/actions/runs/19059073415/job/54435056978?pr=5135#step:6:11

/Users/runner/hostedtoolcache/Ruby/2.7.8/arm64/lib/ruby/2.7.0/singleton.rb:162:in `private_class_method': undefined method `new' for class `#<Class:Fluent::Compat::NullOutputChain>' (NameError)
Did you mean?  next
	from /Users/runner/hostedtoolcache/Ruby/2.7.8/arm64/lib/ruby/2.7.0/singleton.rb:162:in `included'
	from /Users/runner/work/fluentd/fluentd/lib/fluent/compat/output_chain.rb:23:in `include'
	from /Users/runner/work/fluentd/fluentd/lib/fluent/compat/output_chain.rb:23:in `<class:NullOutputChain>'
	from /Users/runner/work/fluentd/fluentd/lib/fluent/compat/output_chain.rb:22:in `<module:Compat>'
	from /Users/runner/work/fluentd/fluentd/lib/fluent/compat/output_chain.rb:20:in `<module:Fluent>'
	from /Users/runner/work/fluentd/fluentd/lib/fluent/compat/output_chain.rb:19:in `<top (required)>'
	from /Users/runner/hostedtoolcache/Ruby/2.7.8/arm64/lib/ruby/site_ruby/2.7.0/rubygems/core_ext/
...

It seems that something changed in Ruby 2.7 or gems, similar errors occur on all platforms with Ruby 2.7.
But we don't need Ruby v2.7 support even in v1.16, we can simply drop CI for Ruby 2.7 in v1.16 branch.
No need wasting our time to investigate it.

https://github.com/fluent/fluentd/actions/runs/19059073415/job/54435056998?pr=5135#step:6:4849

 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

Although I'm not sure the reason yet, same error occurs in #5137 too.
https://github.com/fluent/fluentd/actions/runs/19060858832/job/54440144205?pr=5137#step:6:4753

[Watson1978]

Thanks.