[Rails5] Add `rails-controller-testing` gem

In Rails 5.0 `assert_template` and `assigns()` have been extracted to
the `rails-controller-testing` gem.
https://github.com/rails/rails/pull/20138/files

This commit adds this gem for rails5.

Gemfile

@@ -384,6 +384,7 @@ group :test do
   gem 'email_spec', '~> 1.6.0'
   gem 'json-schema', '~> 2.8.0'
   gem 'webmock', '~> 2.3.2'
+  gem 'rails-controller-testing' if rails5? # Rails5 only gem.
   gem 'test_after_commit', '~> 1.1' unless rails5? # Remove this gem when migrated to rails 5.0. It's been integrated to rails 5.0.
   gem 'sham_rack', '~> 1.3.6'
   gem 'concurrent-ruby', '~> 1.0.5'

Gemfile.rails5.lock

@@ -678,6 +678,10 @@ GEM
       bundler (>= 1.3.0)
       railties (= 5.0.6)
       sprockets-rails (>= 2.0.0)
+    rails-controller-testing (1.0.2)
+      actionpack (~> 5.x, >= 5.0.1)
+      actionview (~> 5.x, >= 5.0.1)
+      activesupport (~> 5.x)
     rails-deprecated_sanitizer (1.0.3)
       activesupport (>= 4.2.0.alpha)
     rails-dom-testing (2.0.3)
@@ -1145,6 +1149,7 @@ DEPENDENCIES
   rack-oauth2 (~> 1.2.1)
   rack-proxy (~> 0.6.0)
   rails (= 5.0.6)
+  rails-controller-testing
   rails-deprecated_sanitizer (~> 1.0.3)
   rails-i18n (~> 5.1)
   rainbow (~> 2.2)