test: handle invalid_grant in default creds system tests

This change updates the system tests for default credentials to explicitly
allow "invalid_grant" errors when running with `authorized_user.json`
credentials. This is because the CI environment uses credentials that
frequently expire, and the server response for expired credentials
has changed to return "invalid_grant", causing tests to fail instead
of being suppressed as intended. This ensures the tests verify the
library correctly handles the expiration error rather than failing.

References: #1882

Author: google-labs-jules[bot]