Add spec for errors rake task

This will prevent re-occurence of today's error where the site
didn't have custom error pages for a few minutes.

Author: pixeltrix

spec/support/rake_task_example_group.rb

@@ -0,0 +1,44 @@
+require 'rake'
+
+module RakeTaskExampleGroup
+  extend ActiveSupport::Concern
+
+  included do
+    let(:rake) { Rake.application }
+    let(:task) { self.class.top_level_description }
+    let(:prerequisites) { subject.prerequisites }
+
+    subject { rake[task] }
+
+    around do |example|
+      with_rake_env { example.run }
+    end
+  end
+
+  private
+
+  def application_tasks
+    Rails.application.paths['lib/tasks'].to_a
+  end
+
+  def with_rake_env
+    new_rake = Rake::Application.new
+    old_rake, Rake.application = Rake.application, new_rake
+
+    # The Rails enviroment is already loaded so we define an
+    # empty environment task to fufill the prerequisites.
+    Rake::Task.define_task(:environment)
+
+    # Load just the application tasks defined in `lib/tasks`
+    application_tasks.each { |task| load(task) }
+
+    yield
+
+  ensure
+    Rake.application = old_rake
+  end
+end
+
+RSpec.configure do |config|
+  config.include RakeTaskExampleGroup, type: :rake, file_path: %r[spec/tasks]
+end

spec/tasks/assets_spec.rb

@@ -0,0 +1,7 @@
+require 'rails_helper'
+
+RSpec.describe "assets:precompile", type: :task do
+  it "includes 'errors:precompile' in its prerequisites" do
+    expect(prerequisites).to include('errors:precompile')
+  end
+end

spec/tasks/errors_spec.rb

@@ -0,0 +1,27 @@
+require 'rails_helper'
+
+RSpec.describe "errors:precompile", type: :task do
+  let(:public_path) { Pathname.new(Dir.mktmpdir) }
+  let(:public_files) { public_path.entries.map(&:to_s) }
+
+  before do
+    allow(Rails).to receive(:public_path).and_return(public_path)
+
+    subject.invoke
+  end
+
+  after do
+    FileUtils.remove_entry(public_path)
+  end
+
+  it "renders the error pages" do
+    expect(public_files).to include('400.html')
+    expect(public_files).to include('403.html')
+    expect(public_files).to include('404.html')
+    expect(public_files).to include('406.html')
+    expect(public_files).to include('422.html')
+    expect(public_files).to include('500.html')
+    expect(public_files).to include('503.html')
+    expect(public_files).to include('error.css')
+  end
+end