Can't add custom field to form
There are multiple ways to do this. One is to use a .extra.tpl which has to have the same path and first part of the name as the original .tpl where you want it to get inserted. If you name it that way the tpl file will get picked up automatically. So in this case the original .tpl that controls that event screen is in templates/CRM/Event/Form/ManageEvent/EventInfo.tpl, so you want to make a file templates/CRM/Event/Form/ManageEvent/EventInfo.extra.tpl

Another way is like in your original question but then you're missing the line from the docs (https://docs.civicrm.org/dev/en/latest/hooks/hook_civicrm_buildForm/) that says

CRM_Core_Region::instance('page-body')-&gt;add(array(
  'template' =&gt; "{$templatePath}/testfield.tpl"
));


to tell it the path and name of your template file. Otherwise it doesn't know about your .tpl file.