Fix Action Mailbox tests

ActiveSupport::TestCase isn't defined until later in the file, so the
skip override can be added at the end to ensure it exists (similar to
what the Action Pack tests do)

Author: skipkayhil

actionmailbox/test/test_helper.rb

@@ -5,8 +5,6 @@
 ENV["RAILS_ENV"] = "test"
 ENV["RAILS_INBOUND_EMAIL_PASSWORD"] = "tbsy84uSV1Kt3ZJZELY2TmShPRs91E3yL4tzf96297vBCkDWgL"
 
-require_relative "../../tools/test_common"
-
 require_relative "../test/dummy/config/environment"
 ActiveRecord::Migrator.migrations_paths = [File.expand_path("../test/dummy/db/migrate", __dir__)]
 require "rails/test_help"
@@ -55,3 +53,5 @@ def bounce(to:)
     end
   end
 end
+
+require_relative "../../tools/test_common"