Make validates cleanly spec compatible with RubyGems 3.5+

`rubygems/mock_gem_ui` was removed in RubyGems 3.5+:

rubygems/rubygems#6623

Luckily, other PR allowed to use standard RSpec methods for `$stdout`
testing:

rubygems/rubygems#7203

Fixes enkessler#190

spec/childprocess_spec.rb

@@ -1,7 +1,6 @@
 # encoding: utf-8
 
 require File.expand_path('../spec_helper', __FILE__)
-require 'rubygems/mock_gem_ui'
 
 
 describe ChildProcess do
@@ -11,12 +10,17 @@
   let(:gemspec) { eval(File.read "#{here}/../childprocess.gemspec") }
 
   it 'validates cleanly' do
-    mock_ui = Gem::MockGemUi.new
-    Gem::DefaultUserInteraction.use_ui(mock_ui) { gemspec.validate }
+    if Gem::VERSION >= "3.5.0"
+      expect { gemspec.validate }.not_to output(/warn/i).to_stderr
+    else
+      require 'rubygems/mock_gem_ui'
 
-    expect(mock_ui.error).to_not match(/warn/i)
-  end
+      mock_ui = Gem::MockGemUi.new
+      Gem::DefaultUserInteraction.use_ui(mock_ui) { gemspec.validate }
 
+      expect(mock_ui.error).to_not match(/warn/i)
+    end
+  end
 
   it "returns self when started" do
     process = sleeping_ruby