A lot :)

config/environments/test.rb

@@ -1,4 +1,4 @@
-require 'active_support/core_ext/integer/time'
+require "active_support/core_ext/integer/time"
 
 # The test environment is used exclusively to run your application's
 # test suite. You never need to work with it otherwise. Remember that
@@ -15,13 +15,10 @@ Rails.application.configure do
   # this is usually not necessary, and can slow down your test suite. However, it's
   # recommended that you enable it in continuous integration systems to ensure eager
   # loading is working properly before deploying your code.
-  config.eager_load = ENV['CI'].present?
+  config.eager_load = ENV["CI"].present?
 
   # Configure public file server for tests with Cache-Control for performance.
-  config.public_file_server.enabled = true
-  config.public_file_server.headers = {
-    'Cache-Control' => "public, max-age=#{1.hour.to_i}"
-  }
+  config.public_file_server.headers = { "Cache-Control" => "public, max-age=#{1.hour.to_i}" }
 
   # Show full error reports and disable caching.
   config.consider_all_requests_local = true
@@ -37,6 +34,8 @@ Rails.application.configure do
   # Store uploaded files on the local file system in a temporary directory.
   config.active_storage.service = :test
 
+  # Disable caching for Action Mailer templates even if Action Controller
+  # caching is enabled.
   config.action_mailer.perform_caching = false
 
   # Tell Action Mailer not to deliver emails to the real world.
@@ -44,6 +43,10 @@ Rails.application.configure do
   # ActionMailer::Base.deliveries array.
   config.action_mailer.delivery_method = :test
 
+  # Unlike controllers, the mailer instance doesn't have any context about the
+  # incoming request so you'll need to provide the :host parameter yourself.
+  config.action_mailer.default_url_options = { host: "www.example.com" }
+
   # Print deprecation notices to the stderr.
   config.active_support.deprecation = :stderr
 
@@ -59,6 +62,6 @@ Rails.application.configure do
   # Annotate rendered view with file names.
   # config.action_view.annotate_rendered_view_with_filenames = true
 
-  # Raise error when a before_action's only/except options reference missing actions
+  # Raise error when a before_action's only/except options reference missing actions.
   config.action_controller.raise_on_missing_callback_actions = true
 end