Added integration test for attachment deletion

This demonstrates the current behaviour when an attachment is deleted
from a draft edition which has never been published.

* `Admin::AttachmentsController#destroy` calls `Attachment#destroy`.

* `Attachment#destroy` overrides the standard
`ActiveRecord::Persistence#destroy` method so that it only marks the
attachment as deleted, but it still runs `:destroy` callbacks [1].

* `FileAttachment`, a subclass of `Attachment` has an `after_destroy`
callback which will destroy the associated `AttachmentData` only if
no other non-deleted attachments are using it [2].

* In this case, because there's only a single edition of the document,
there is only one instance of `Attachment` for the relevant
`AttachmentData` and thus the latter is destroyed.

* This in turn fires an `after_commit, on: :destroy` callback which is
added by CarrierWave which eventually results in a call to
`Whitehall::AssetManagerStorage::File#delete` which calls
`AssetManagerDeleteAssetWorker.perform_async` to queue a job to delete
the corresponding asset in Asset Manager.

* The fact that the `AttachmentData` has actually been deleted results
in an `ActiveRecord::RecordNotFound` exception being raised in
`AttachmentsController#attachment_data`. In production this will result
in a `404 Not Found` response [3].

* In the case where an `AttachmentData` is associated with more than one
edition, the deletion of an `Attachment` will only result in the
`Attachment` being marked as deleted; the `AttachmentData` will not be
destroyed`. However, requests to the attachment URL will still result in
a `404 Not Found`, because `AttachmentVisibility#edition_ids` [4] does
not consider deleted attachments and thus
`AttachmentVisibility#visible?` will always return `false` and this line
[5] in `AttachmentsController#fail` will be invoked.

[1]:
https://github.com/alphagov/whitehall/blob/32bb13cf3ae3ae9ca71fa14bd4c620c631660419/app/models/attachment.rb#L130-L132
[2]:
https://github.com/alphagov/whitehall/blob/32bb13cf3ae3ae9ca71fa14bd4c620c631660419/app/models/file_attachment.rb#L33-L38
[3]:
http://guides.rubyonrails.org/v5.0.6/action_controller_overview.html#rescue
[4]:
https://github.com/alphagov/whitehall/blob/32bb13cf3ae3ae9ca71fa14bd4c620c631660419/app/models/attachment_visibility.rb#L116-L118
[5]:
https://github.com/alphagov/whitehall/blob/32bb13cf3ae3ae9ca71fa14bd4c620c631660419/app/controllers/attachments_controller.rb#L31

test/integration/attachment_deletion_integration_test.rb

@@ -0,0 +1,82 @@
+require 'test_helper'
+require 'capybara/rails'
+
+class AttachmentDeletionIntegrationTest < ActionDispatch::IntegrationTest
+  extend Minitest::Spec::DSL
+  include Capybara::DSL
+  include Rails.application.routes.url_helpers
+
+  context 'given a draft document with a file attachment' do
+    let(:managing_editor) { create(:managing_editor) }
+
+    let(:filename) { 'sample.docx' }
+    let(:file) { File.open(path_to_attachment(filename)) }
+    let(:attachment) { build(:file_attachment, attachable: edition, file: file) }
+    let(:asset_id) { 'asset-id' }
+
+    let(:replacement_filename) { 'sample.rtf' }
+    let(:replacement_asset_id) { 'replacement-asset-id' }
+
+    let(:edition) { create(:news_article) }
+
+    before do
+      login_as(managing_editor)
+      edition.attachments << attachment
+      setup_publishing_api_for(edition)
+      stub_whitehall_asset(filename, id: asset_id)
+      VirusScanHelpers.simulate_virus_scan
+
+      visit admin_news_article_path(edition)
+      click_link 'Modify attachments'
+      @attachment_url = find('.existing-attachments a', text: filename)[:href]
+    end
+
+    context 'when attachment is deleted' do
+      before do
+        visit admin_news_article_path(edition)
+        click_link 'Modify attachments'
+        within '.existing-attachments' do
+          click_link 'Delete'
+        end
+      end
+
+      it 'responds with 404 Not Found for attachment URL' do
+        logout
+
+        assert_raises(ActiveRecord::RecordNotFound) do
+          get @attachment_url
+        end
+      end
+
+      it 'deletes attachment in Asset Manager' do
+        Services.asset_manager.expects(:delete_asset)
+          .with(asset_id)
+        AssetManagerDeleteAssetWorker.drain
+      end
+    end
+  end
+
+private
+
+  def ends_with(expected)
+    ->(actual) { actual.end_with?(expected) }
+  end
+
+  def setup_publishing_api_for(edition)
+    publishing_api_has_links(
+      content_id: edition.document.content_id,
+      links: {}
+    )
+  end
+
+  def path_to_attachment(filename)
+    fixture_path.join(filename)
+  end
+
+  def stub_whitehall_asset(filename, attributes = {})
+    url_id = "http://asset-manager/assets/#{attributes[:id]}"
+    Services.asset_manager.stubs(:whitehall_asset)
+      .with(&ends_with(filename))
+      .returns(attributes.merge(id: url_id).stringify_keys)
+  end
+end