test_plugin_helper: fix test class name

[Watson1978]

Which issue(s) this PR fixes:
Fixes #

What this PR does / why we need it:

test_config.rb and test_plugin_helper.rb has same class name.

fluentd/test/test_config.rb

Line 8 in 3380500

class ConfigTest < Test::Unit::TestCase

fluentd/test/test_plugin_helper.rb

Line 5 in 3380500

class ConfigTest < Test::Unit::TestCase

The test class name should be unique.

Docs Changes:
Not needed.

Release Note:
CI improvements.

[Watson1978]

Seems ConfigTest class appears to hold test cases for test_config.rb and test_plugin_helper.rb in one place.

bundle exec ruby -w -Ilib -Itest C:/Ruby33-x64/lib/ruby/gems/3.3.0/gems/rake-13.3.0/lib/rake/rake_test_loader.rb test/test_config.rb test/test_plugin_helper.rb -v
C:/Ruby33-x64/lib/ruby/gems/3.3.0/gems/serverengine-2.4.0/lib/serverengine/socket_manager_win.rb:141: warning: assigned but unused variable - type
C:/Ruby33-x64/lib/ruby/gems/3.3.0/gems/protocol-http-0.52.0/lib/protocol/http/header/accept.rb:84: warning: parentheses after method name is interpreted as an argument list, not a decomposed argument
C:/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: method redefined; discarding old inherited
C:/src/fluentd/lib/fluent/plugin_helper.rb:46: warning: previous definition of inherited was here
Loaded suite C:/Ruby33-x64/lib/ruby/gems/3.3.0/gems/rake-13.3.0/lib/rake/rake_test_loader
Started
ConfigTest: 
  test_check_not_fetchd:																			.: (0.001557)
  test_include:																					.: (0.006763)
  .build: 
    test: read config:																				.: (0.001474)
    test: read config with additional_config:																	.: (0.000867)
    test: read config with encoding:																		.: (0.005180)
  Fluent::Plugin::Base.helpers method works as shortcut to include helper modules: 
    test: plugin can 2 or more helpers at once:																	.: (0.000450)
    test: plugin can include helper child_process:																.: (0.000191)
    test: plugin can include helper event_emitter:																.: (0.000156)
    test: plugin can include helper event_loop:																	.: (0.000130)
    test: plugin can include helper thread:																	.: (0.000194)
    test: plugin can include helper timer:																	.: (0.000120)
  yaml config: 
    test_check_not_fetchd:																			.: (0.001134)
    test_included:																				.: (0.001920)
    test_included_glob:						

[daipom]

Thanks!