pass form parameter to hook by reference

Problem/Motivation

In the current dev version, phpstan is reporting the following error in WebformTestAlterHooksHooks.php:

Form alter hook "form_webform_submission_form_alter" first parameter must be passed by reference (&$form). Expected signature: method(&$form, \Drupal\Core\Form\FormStateInterface $form_state[, $form_id])

Proposed resolution

pass the parameter by reference, as directed.

Remaining tasks

make an MR. I'm assigning this to myself since I intend to work on it right away.