[ruby/logger] Fix to use logger and test-unit in this repo with

`ruby test/logger/test_xxx.rb`

https://github.com/ruby/logger/commit/d3c2402340

test/helper.rb

@@ -0,0 +1,5 @@
+ROOT_DIR = File.dirname(__dir__)
+$LOAD_PATH.unshift File.join(ROOT_DIR, 'lib') # to use logger in this repo instead of ruby built-in logger
+$LOAD_PATH.unshift File.join(ROOT_DIR, 'test', 'lib') # to use custom test-unit in this repo
+require 'logger'
+require 'test/unit'

test/logger/test_logdevice.rb

@@ -1,7 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require 'test/unit'
-require 'logger'
+require_relative '../helper'
 require 'tempfile'
 require 'tmpdir'
 

test/logger/test_logger.rb

@@ -1,7 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require 'test/unit'
-require 'logger'
+require_relative '../helper'
 require 'tempfile'
 
 class TestLogger < Test::Unit::TestCase

test/logger/test_severity.rb

@@ -1,7 +1,6 @@
 # coding: US-ASCII
 # frozen_string_literal: false
-require 'test/unit'
-require 'logger'
+require_relative '../helper'
 
 class TestLoggerSeverity < Test::Unit::TestCase
   def test_enum
@@ -13,13 +12,13 @@ class TestLoggerSeverity < Test::Unit::TestCase
     end
     assert_equal(levels.size, Logger::Severity.constants.size)
   end
-  
+
   def test_level_assignment
     logger = Logger.new(nil)
-    
+
     Logger::Severity.constants.each do |level|
       next if level == :UNKNOWN
-      
+
       logger.send("#{level.downcase}!")
       assert(logger.level) == Logger::Severity.const_get(level)
     end