MDL-63185 core_test: add set_user function to behat

Part of MDL-62610

lib/behat/behat_base.php

@@ -946,6 +946,32 @@ class behat_base extends Behat\MinkExtension\Context\RawMinkContext {
         return $DB->get_record('user', ['id' => $userid]);
     }
 
+    /**
+     * Set current $USER, reset access cache.
+     * @static
+     * @param null|int|stdClass $user user record, null or 0 means non-logged-in, positive integer means userid
+     * @return void
+     */
+    public static function set_user($user = null) {
+        global $DB;
+
+        if (is_object($user)) {
+            $user = clone($user);
+        } else if (!$user) {
+            // Assign valid data to admin user (some generator-related code needs a valid user).
+            $user = $DB->get_record('user', array('username' => 'admin'));
+        } else {
+            $user = $DB->get_record('user', array('id'=> $user));
+        }
+        unset($user->description);
+        unset($user->access);
+        unset($user->preference);
+
+        // Enusre session is empty, as it may contain caches and user specific info.
+        \core\session\manager::init_empty_session();
+
+        \core\session\manager::set_user($user);
+    }
     /**
      * Trigger click on node via javascript instead of actually clicking on it via pointer.
      *