added cron_core to fixtures for puppet 6
added cron_core to fixtures for puppet 6

puppetlabs/puppetlabs-mysql,DavidS/puppetlabs-mysql,markasammut/puppetlabs-mysql,simp/puppetlabs-mysql,puppetlabs/puppetlabs-mysql,markasammut/puppetlabs-mysql,simp/puppetlabs-mysql,DavidS/puppetlabs-mysql