Remove hard dependency on test-unit

Instead show a error message asking users to add the gem to their
Gemfile if test-unit could not be loaded.

Gemfile

@@ -18,6 +18,7 @@ else
 end
 
 gem 'i18n', '~> 0.6.11'
+gem 'test-unit',  '~> 3.0'
 
 # This needs to be with require false to avoid
 # it being automatically loaded by sprockets

activesupport/activesupport.gemspec

@@ -22,5 +22,4 @@ Gem::Specification.new do |s|
 
   s.add_dependency('i18n',       '~> 0.6', '>= 0.6.4')
   s.add_dependency('multi_json', '~> 1.0')
-  s.add_dependency('test-unit',  '~> 3.0')
 end

activesupport/lib/active_support/test_case.rb

@@ -1,4 +1,8 @@
-require 'test/unit/testcase'
+begin
+  require 'test/unit/testcase'
+rescue LoadError => e
+  raise LoadError, "Please add test-unit gem to your Gemfile: `gem 'test-unit', '~> 3.0'` (#{e.message})", e.backtrace
+end
 require 'active_support/testing/setup_and_teardown'
 require 'active_support/testing/assertions'
 require 'active_support/testing/deprecation'

activesupport/lib/active_support/testing/isolation.rb

@@ -156,7 +156,12 @@ def run_in_isolation(&blk)
 
 # Only in subprocess for windows / jruby.
 if ENV['ISOLATION_TEST']
-  require "test/unit/collector/objectspace"
+  begin
+    require "test/unit/collector/objectspace"
+  rescue LoadError => e
+    raise LoadError, "Please add test-unit gem to your Gemfile: `gem 'test-unit', '~> 3.0'` (#{e.message})", e.backtrace
+  end
+
   class Test::Unit::Collector::ObjectSpace
     def include?(test)
       super && test.method_name == ENV['ISOLATION_TEST']

railties/lib/rails/test_help.rb

@@ -2,7 +2,11 @@
 # so fixtures aren't loaded into that environment
 abort("Abort testing: Your Rails environment is running in production mode!") if Rails.env.production?
 
-require 'test/unit'
+begin
+  require 'test/unit'
+rescue LoadError => e
+  raise LoadError, "Please add test-unit gem to your Gemfile: `gem 'test-unit', '~> 3.0'` (#{e.message})", e.backtrace
+end
 require 'active_support/test_case'
 require 'action_controller/test_case'
 require 'action_dispatch/testing/integration'