test_plugin_helper: fix test class name (#5064)

**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.


https://github.com/fluent/fluentd/blob/338050012279f401af41c95049d82cb98b2a370b/test/test_config.rb#L8


https://github.com/fluent/fluentd/blob/338050012279f401af41c95049d82cb98b2a370b/test/test_plugin_helper.rb#L5

The test class name should be unique.

**Docs Changes**:
Not needed.

**Release Note**: 
CI improvements.

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

Author: Watson1978

test/test_plugin_helper.rb

@@ -2,7 +2,7 @@
 require 'fluent/plugin_helper'
 require 'fluent/plugin/base'
 
-class ConfigTest < Test::Unit::TestCase
+class PluginHelperTest < Test::Unit::TestCase
   module FluentTest; end
 
   sub_test_case 'Fluent::Plugin::Base.helpers method works as shortcut to include helper modules' do