Let the statisticsForms model return the total signups instead of the form model

- added unit test for getTotalSignups() method

lib/Models/StatisticsForms.php

@@ -6,6 +6,10 @@ if(!defined('ABSPATH')) exit;
 class StatisticsForms extends Model {
   public static $_table = MP_STATISTICS_FORMS_TABLE;
 
+  public static function getTotalSignups($form_id = false) {
+    return self::where('form_id', $form_id)->count();
+  }
+
   public static function record($form_id, $subscriber_id) {
     if($form_id > 0 && $subscriber_id > 0) {
       // check if we already have a record for today