MDL-77337 analytics: Added class properties that are not declared.

In PHP 8.2 and later, setting a value to an undeclared class property is
deprecated and emits a deprecation notice.
So we need to add missing class properties that still need to be declared.

analytics/tests/model_test.php

@@ -43,6 +43,12 @@ require_once(__DIR__ . '/fixtures/test_analysis.php');
  */
 class model_test extends \advanced_testcase {
 
+    /** @var model Store Model. */
+    protected $model;
+
+    /** @var \stdClass Store model object. */
+    protected $modelobj;
+
     public function setUp(): void {
 
         $this->setAdminUser();