How override the default form template ?

I generate a simple module "myformtest" with a simple Class form "MyformtestForm". I want override the default form template and pass to the template a custom array generated from my build method of form Class.

I followed this great tutorial . I added a "_theme_suggestions_alter" hook in my "myformtest.module", I also added the hook theme.

My "suggestion theme" is "form--myformtest-form.html.twig". Then in my module directory:

- I added a sub-directory "templates"
- I added the twig file "form--myformtest-form.html.twig" in the sub-directory "drupal/modules/custom/myformtest/templates"

In my .twig file, I added this simple code. (I copied it from the drupal/core/themes/classy/templates/form/form.html.twig and added the h1 html element for override)

    <h1>TEST</h1>
    <form{{ attributes }}>
        {{ children }}
    </form>

I clear caches (drush cr) and I reloaded my form page. The html element h1 "test" is displayed but not the form :/ I thinked my .twig file override the form.html.twig default theme...

form page link image

Did I miss something ? Why the form isn't displayed, how can I fix it ? And how can I pass a custom array from my build method of form Class to my twig template ?

Below, the code of my .module file :

/**
 * Implements hook_theme_suggestions_alter().
 */
function myformtest_theme_suggestions_alter(array &$suggestions, array $variables, $hook)
{
  if (isset($variables['element']) && isset($variables['element']['#type']) && $variables['element']['#type'] == 'form') {
    $original_theme_hook = $variables['theme_hook_original'];
    array_unshift($suggestions, $original_theme_hook . '__' . str_replace('-', '_', $variables['element']['#id']));
  }
}

/**
 * Implements hook_theme().
 */
function myformtest_theme($existing, $type, $theme, $path)
{
  // if I comment this function, the form is displayed (but without the html element h1 "test")
  return array(
    'form__myformtest_form' => array(
      'render element' => 'form',
      'template' => 'form--myformtest-form',
    ),
  );
}

If I comment the myformtest_theme function, the form is displayed (but without the html element h1 "test") :

form page link image

Thanks for help :)