tests: journalist: add assert to test_delete_source_deletes_submissions
tests: journalist: add assert to test_delete_source_deletes_submissions

micahflee/securedrop,garrettr/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,micahflee/securedrop,micahflee/securedrop,garrettr/securedrop,heartsucker/securedrop,heartsucker/securedrop,micahflee/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,conorsch/securedrop,garrettr/securedrop,conorsch/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,garrettr/securedrop