Add assertions on real-time feature spec

Fixes a flaky spec when Capybara closes the browser before the
ActionCable subscription is established

spec/features/action_cable_logging_spec.rb

@@ -33,5 +33,15 @@ RSpec.describe 'ActionCable logging', :js do
 
     gitlab_sign_in(user)
     visit project_issue_path(project, issue)
+
+    # Because there is no visual indicator for Capybara to wait on before closing the browser,
+    # we need to test an actual feature to ensure that the subscription was already established.
+
+    expect(page.find('.assignee')).to have_content 'None'
+
+    fill_in 'note[note]', with: "/assign #{user.username}"
+    click_button 'Comment'
+
+    expect(page.find('.assignee')).to have_content user.name
   end
 end