Check the DOM after attaching file has succeeded

We were checking `expect_document_has_title(0, "My Title")`, which was
already true before the AJAX request generated by `attach_file` had
finished.

That meant the AJAX request sometimes was handled after this test had
finished, affecting the following test and causing it to fail because
its cookie was overwritten and so `current_user` was set to `nil`.

In the test checking the filename is present, a similar scenario was
taking place: we were updating the `.file-name` element in the `change`
event of `fileupload` (using `App.Documentable.setFilename`); that is,
when the AJAX request started. And so the test passed before the request
was finished, causing the same issue.

spec/shared/system/nested_documentable.rb

@@ -80,6 +80,8 @@ shared_examples "nested documentable" do |login_as_name, documentable_factory_na
           Rails.root.join("spec/fixtures/files/empty.pdf"),
           make_visible: true
         )
+
+        expect(page).to have_css ".loading-bar.complete"
       end
 
       expect(page).to have_css ".file-name", text: "empty.pdf"
@@ -110,6 +112,8 @@ shared_examples "nested documentable" do |login_as_name, documentable_factory_na
           Rails.root.join("spec/fixtures/files/empty.pdf"),
           make_visible: true
         )
+
+        expect(page).to have_css ".loading-bar.complete"
       end
 
       expect_document_has_title(0, "My Title")