Update 'rake initializers' to use Rails::Command under the hood

* Invoke Rails::Command within the rake task
* Adds test for calling initializers with 'bin/rake'
* Adds deprecation warning to the rake task

railties/lib/rails/tasks/initializers.rake

@@ -1,8 +1,10 @@
 # frozen_string_literal: true
 
+require "rails/command"
+require "active_support/deprecation"
+
 desc "Print out all defined initializers in the order they are invoked by Rails."
-task initializers: :environment do
-  Rails.application.initializers.tsort_each do |initializer|
-    puts "#{initializer.context_class}.#{initializer.name}"
-  end
+task :initializers do
+  ActiveSupport::Deprecation.warn("Using `bin/rake initializers` is deprecated and will be removed in Rails 6.1. Use `bin/rails initializers` instead.\n")
+  Rails::Command.invoke "initializers"
 end

railties/test/application/rake/initializers_test.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require "isolation/abstract_unit"
+
+module ApplicationTests
+  module RakeTests
+    class RakeInitializersTest < ActiveSupport::TestCase
+      setup :build_app
+      teardown :teardown_app
+
+      test "`rake initializers` prints out defined initializers invoked by Rails" do
+        capture(:stderr) do
+          initial_output = run_rake_initializers
+          initial_output_length = initial_output.split("\n").length
+
+          assert_operator initial_output_length, :>, 0
+          assert_not initial_output.include?("set_added_test_module")
+
+          add_to_config <<-RUBY
+            initializer(:set_added_test_module) { }
+          RUBY
+
+          final_output = run_rake_initializers
+          final_output_length = final_output.split("\n").length
+
+          assert_equal 1, (final_output_length - initial_output_length)
+          assert final_output.include?("set_added_test_module")
+        end
+      end
+
+      test "`rake initializers` outputs a deprecation warning" do
+        stderr = capture(:stderr) { run_rake_initializers }
+        assert_match(/DEPRECATION WARNING: Using `bin\/rake initializers` is deprecated and will be removed in Rails 6.1/, stderr)
+      end
+
+      private
+        def run_rake_initializers
+          Dir.chdir(app_path) { `bin/rake initializers` }
+        end
+    end
+  end
+end