use DrupalCoreFormFormStateInterface;
use SymfonyComponentHttpFoundationRequest;

function testmodule_form_node_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  foreach (array_keys($form['tasks']) as $action) {
    if ($action != 'preview' && isset($form['tasks'][$action]['#type']) && $form['tasks'][$action]['#type'] === 'submit') {
      $form['tasks'][$action]['#submit'][] = 'testmodule_form_submit';
    }
  }
}


function testmodule_form_submit(array $form, FormStateInterface $form_state){
    drupal_set_message("Why won't this message show?");
}