MDL-65518 badges: Retry auth test

When testing the site backpack login credentials, don't rely on a previous login
because the username and password could have changed.

badges/classes/backpack_api.php

@@ -473,6 +473,21 @@ public function put_issuer($data) {
         return $this->curl_request('issuers', null, null, $data);
     }
 
+    /**
+     * Delete any user access tokens in the session so we will attempt to get new ones.
+     *
+     * @return void
+     */
+    public function clear_system_user_session() {
+        global $SESSION;
+
+        $useridkey = $this->get_token_key(BADGE_USER_ID_TOKEN);
+        unset($SESSION->$useridkey);
+
+        $expireskey = $this->get_token_key(BADGE_EXPIRES_TOKEN);
+        unset($SESSION->$expireskey);
+    }
+
     /**
      * Authenticate using the stored email and password and save the valid access tokens.
      *

lib/badgeslib.php

@@ -1173,7 +1173,8 @@ function badge_assemble_notification(stdClass $badge) {
 
 /**
  * Attempt to authenticate with the site backpack credentials and return an error
- * if the authentication fails.
+ * if the authentication fails. If external backpacks are not enabled, this will
+ * not perform any test.
  *
  * @return string
  */
@@ -1189,6 +1190,10 @@ function badges_verify_site_backpack() {
     if (empty($backpack->apiversion) || ($backpack->apiversion == OPEN_BADGES_V2)) {
         $backpackapi = new \core_badges\backpack_api($backpack);
 
+        // Clear any cached access tokens in the session.
+        $backpackapi->clear_system_user_session();
+
+        // Now attempt a login with these credentials.
         $result = $backpackapi->authenticate();
         if ($result === false || !empty($result->error)) {
             $warning = $backpackapi->get_authentication_error();