How to test whenever config

I recently made a typo in the whenever (Ruby cron jobs manager) — config:

# config/schedule.rb
every 1.day, at: "03:30 am", roles: %i(backupable) do
  rake %( # %( should be %w(
    backup:db
    backup:assets
  ).join(" ")
end

It broke deployment pipeline with the following error:

NoMethodError: undefined method `join' for "backup:db backup:assets":String

To avoid this in the future, we need at least minimal validation of the whenever config. The solution is simple, just run this command on CI:

bundle exec whenever

If there are problems in config, it will return an error. If there are no problems, it will display the resulting crontab.