support minitest after_run for parity to regular minitest

see https://github.com/seattlerb/minitest/blob/f9605387e4af7d657921a83aaf0ae364f6d26a57/lib/minitest.rb#L51-L65

railties/CHANGELOG.md

+*   Run `Minitest.after_run` hooks when running `rails test`.
+
+    *Michael Grosser*
+
 *   Run `before_configuration` callbacks as soon as application constant
     inherits from `Rails::Application`.
 

railties/lib/rails/commands/test.rb

@@ -6,4 +6,6 @@
   $LOAD_PATH << File.expand_path("../../test", APP_PATH)
 end
 
-exit Minitest.run(ARGV)
+result = Minitest.run(ARGV)
+Minitest.class_variable_get(:@@after_run).reverse_each(&:call)
+exit result

railties/test/application/test_test.rb

@@ -26,6 +26,24 @@ def test_truth
       assert_successful_test_run "unit/foo_test.rb"
     end
 
+    test "after_run" do
+      app_file "test/unit/foo_test.rb", <<-RUBY
+        require 'test_helper'
+
+        Minitest.after_run { puts "WORLD" }
+        Minitest.after_run { puts "HELLO" }
+
+        class FooTest < ActiveSupport::TestCase
+          def test_truth
+            assert true
+          end
+        end
+      RUBY
+
+      result = assert_successful_test_run "unit/foo_test.rb"
+      assert_equal ["HELLO", "WORLD"], result.scan(/HELLO|WORLD/) # only once and in correct order
+    end
+
     test "simple failed test" do
       app_file "test/unit/foo_test.rb", <<-RUBY
         require 'test_helper'