use our runner (`bin/test`) for framework components.

This adds a script `bin/test` to most Rails framework components. The
script uses the rails minitest plugin to augment the runner.
See https://github.com/rails/rails/pull/19571 for details about the
plugin.

I did not yet add `bin/test` for activerecord, activejob and railties.
These components rely on specific setup performed in the rake-tasks.

actionmailer/bin/test

+#!/usr/bin/env ruby
+COMPONENT_ROOT = File.expand_path("../../", __FILE__)
+require File.expand_path("../tools/test", COMPONENT_ROOT)

actionpack/bin/test

+#!/usr/bin/env ruby
+COMPONENT_ROOT = File.expand_path("../../", __FILE__)
+require File.expand_path("../tools/test", COMPONENT_ROOT)

actionview/bin/test

+#!/usr/bin/env ruby
+COMPONENT_ROOT = File.expand_path("../../", __FILE__)
+require File.expand_path("../tools/test", COMPONENT_ROOT)

activemodel/bin/test

+#!/usr/bin/env ruby
+COMPONENT_ROOT = File.expand_path("../../", __FILE__)
+require File.expand_path("../tools/test", COMPONENT_ROOT)

activesupport/bin/test

+#!/usr/bin/env ruby
+COMPONENT_ROOT = File.expand_path("../../", __FILE__)
+require File.expand_path("../tools/test", COMPONENT_ROOT)

tools/test.rb

+$: << File.expand_path("test", COMPONENT_ROOT)
+require File.expand_path("../../load_paths", __FILE__)
+require "rails/test_unit/minitest_plugin"
+
+module Rails
+  # Necessary to get rerun-snippts working.
+  def self.root
+    @root ||= Pathname.new(COMPONENT_ROOT)
+  end
+end
+
+exit Minitest.run(ARGV)