[ActiveJob] Autoload adapters

That will fix a bug when a backend don't require the adapter first

fixes: https://github.com/mperham/sidekiq/issues/1912

That will also autoload the TestAdapter when required instead of always requiring it. (WIP)

cc @mperham @dhh @rafaelfranca 
I believe we don't need to use autoload for this. It will be lazy loaded anyway.