[rubygems/rubygems] Use globals variables for standard input/output

Replace use of `STDIN`, `STDOUT` and `STDERR` constants by their
`$stdin`, `$stdout` and `$stderr` global variable equivalents.

This enables easier testing via standard means, such as `assert_output`
for minitest or `expect { print 'foo' }.to output.to_stdout` for RSpec
test suites.

https://github.com/rubygems/rubygems/commit/a0a6fc1b76

lib/rubygems/user_interaction.rb

@@ -193,7 +193,7 @@ class Gem::StreamUI
   # then special operations (like asking for passwords) will use the TTY
   # commands to disable character echo.
 
-  def initialize(in_stream, out_stream, err_stream=STDERR, usetty=true)
+  def initialize(in_stream, out_stream, err_stream=$stderr, usetty=true)
     @ins = in_stream
     @outs = out_stream
     @errs = err_stream
@@ -591,8 +591,8 @@ class Gem::StreamUI
 end
 
 ##
-# Subclass of StreamUI that instantiates the user interaction using STDIN,
-# STDOUT, and STDERR.
+# Subclass of StreamUI that instantiates the user interaction using $stdin,
+# $stdout, and $stderr.
 
 class Gem::ConsoleUI < Gem::StreamUI
   ##
@@ -600,7 +600,7 @@ class Gem::ConsoleUI < Gem::StreamUI
   # stdin, output to stdout and warnings or errors to stderr.
 
   def initialize
-    super STDIN, STDOUT, STDERR, true
+    super $stdin, $stdout, $stderr, true
   end
 end
 

test/rubygems/test_gem_console_ui.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require_relative "helper"
+require "rubygems/user_interaction"
+
+class TestGemConsoleUI < Gem::TestCase
+  def test_output_can_be_captured_by_test_unit
+    output = capture_output do
+      ui = Gem::ConsoleUI.new
+
+      ui.alert_error "test error"
+      ui.alert_warning "test warning"
+      ui.alert "test alert"
+    end
+
+    assert_equal "INFO:  test alert\n", output.first
+    assert_equal "ERROR:  test error\n" + "WARNING:  test warning\n", output.last
+  end
+end