test: Update tests for Newsletter
test: Update tests for Newsletter

Added tests for better coverage: Increased to 77% ~ near complete
coverage of controller class, missing tests for API endpoints

frappe/frappe,StrellaGroup/frappe,yashodhank/frappe,yashodhank/frappe,StrellaGroup/frappe,frappe/frappe,mhbu50/frappe,mhbu50/frappe,mhbu50/frappe,almeidapaulopt/frappe,StrellaGroup/frappe,frappe/frappe,almeidapaulopt/frappe,yashodhank/frappe,almeidapaulopt/frappe,mhbu50/frappe,almeidapaulopt/frappe,yashodhank/frappe