[ruby/irb] Don't insert new methods to Test::Unit::TestCase

Ruby CI runs irb and other Ruby core/stdlib tests in the same process.
So adding irb-specific helper to Test::Unit::TestCase could potentially
pollute other components' tests and should be avoided.

test/irb/test_cmd.rb

@@ -3,6 +3,8 @@
 require "irb"
 require "irb/extend-command"
 
+require_relative "test_helper"
+
 module TestIRB
   class ExtendCommand < Test::Unit::TestCase
     class TestInputMethod < ::IRB::InputMethod
@@ -443,7 +445,7 @@ def test_help_without_rdoc
       irb = IRB::Irb.new(IRB::WorkSpace.new(self), input)
       IRB.conf[:MAIN_CONTEXT] = irb.context
       out, err = capture_output do
-        without_rdoc do
+        IRB::TestHelper.without_rdoc do
           irb.eval_input
         end
       end

test/irb/test_helper.rb

@@ -0,0 +1,16 @@
+module IRB
+  module TestHelper
+    def self.without_rdoc(&block)
+      ::Kernel.send(:alias_method, :old_require, :require)
+
+      ::Kernel.define_method(:require) do |name|
+        raise LoadError, "cannot load such file -- rdoc (test)" if name.match?("rdoc") || name.match?(/^rdoc\/.*/)
+        ::Kernel.send(:old_require, name)
+      end
+
+      yield
+    ensure
+      EnvUtil.suppress_warning { ::Kernel.send(:alias_method, :require, :old_require) }
+    end
+  end
+end

test/irb/test_input_method.rb

@@ -3,6 +3,8 @@
 require "test/unit"
 require "irb"
 
+require_relative "test_helper"
+
 module TestIRB
   class TestRelineInputMethod < Test::Unit::TestCase
     def setup
@@ -76,7 +78,7 @@ def test_initialization_with_use_autocomplete_but_without_rdoc
 
       IRB.conf[:USE_AUTOCOMPLETE] = true
 
-      without_rdoc do
+      IRB::TestHelper.without_rdoc do
         IRB::RelineInputMethod.new
       end