[ruby/optparse] Make the result of tty? obtainable with flexible stdout

In mock testing for stdout, `StringIO.new` is sometimes used to redirect the output.
In such cases, the assignment is done with `$stdout = StringIO.new`, not the constant `STDOUT`.
e.g., https://github.com/rubocop/rubocop/blob/v1.71.1/lib/rubocop/rspec/shared_contexts.rb#L154-L164

After assigning `StringIO.new`, `$stdout.tty?` returns `false`,
allowing the standard output destination to be switched during test execution.

```ruby
STDOUT.tty?       # => true
StringIO.new.tty? # => false
```

However, since `STDOUT.tty?` returns `true`, a failure occurred in environments
where the environment variables `RUBY_PAGER` or `PAGER` are set.
e.g., rubocop/rubocop#13784

To address this, `STDOUT` has been updated to `$stdout` so that the result of `tty?` can be flexibly overridden.

A potential concern is that `$stdout`, unlike `STDOUT`,
does not always represent the standard output at the time the Ruby process started.
However, no concrete examples of issues related to this have been identified.

`STDOUT.tty?` is the logic of optparse introduced in ruby/optparse#70.

This PR replaces `STDOUT` with `$stdout` throughout, based on the assumption
that `$stdout` is sufficient for use with optparse.

ruby/optparse@262cf6f9ac

Author: koic

lib/optparse.rb

@@ -1056,7 +1056,7 @@ def compsys(to, name = File.basename($0)) # :nodoc:
   end
 
   def help_exit
-    if STDOUT.tty? && (pager = ENV.values_at(*%w[RUBY_PAGER PAGER]).find {|e| e && !e.empty?})
+    if $stdout.tty? && (pager = ENV.values_at(*%w[RUBY_PAGER PAGER]).find {|e| e && !e.empty?})
       less = ENV["LESS"]
       args = [{"LESS" => "#{!less || less.empty? ? '-' : less}Fe"}, pager, "w"]
       print = proc do |f|
@@ -1065,7 +1065,7 @@ def help_exit
         # pager terminated
       end
       if Process.respond_to?(:fork) and false
-        IO.popen("-") {|f| f ? Process.exec(*args, in: f) : print.call(STDOUT)}
+        IO.popen("-") {|f| f ? Process.exec(*args, in: f) : print.call($stdout)}
         # unreachable
       end
       IO.popen(*args, &print)
@@ -1107,7 +1107,7 @@ def help_exit
   #
   Officious['*-completion-zsh'] = proc do |parser|
     Switch::OptionalArgument.new do |arg|
-      parser.compsys(STDOUT, arg)
+      parser.compsys($stdout, arg)
       exit
     end
   end

test/optparse/test_optparse.rb

@@ -184,10 +184,9 @@ def test_help_pager
       File.open(File.join(dir, "options.rb"), "w") do |f|
         f.puts "#{<<~"begin;"}\n#{<<~'end;'}"
         begin;
-          stdout = STDOUT.dup
+          stdout = $stdout.dup
           def stdout.tty?; true; end
-          Object.__send__(:remove_const, :STDOUT)
-          STDOUT = stdout
+          $stdout = stdout
           ARGV.options do |opt|
           end;
           100.times {|i| f.puts "  opt.on('--opt-#{i}') {}"}