Since #50715, there are cases where `ActiveJob::Arguments` is used
but not required anymore in the test environment. This could be a
test for a custom argument serializer.

One solution could be to add a require "active_job/arguments" in
all the usages, but that isn't a convention we follow in Rails.

Instead, let's register the autoload for `ActiveJob::Arguments` in
`active_job.rb` so that it's always available when needed.