Improve error message for older Moodle versions

[ilyatregubov]

if you install MFA for Moodle 3.5 and don't backport anything (i.e. MDL-60470 and MDL-66340) unit tests will explode with error:

Undefined property: stdClass::$mfa_config_hook_test

/var/www/site/admin/tool/mfa/tests/manager_test.php:353
/var/www/site/lib/phpunit/classes/advanced_testcase.php:80

To re-run:
 vendor/bin/phpunit "tool_mfa\tests\tool_mfa_manager_testcase" admin/tool/mfa/tests/manager_test.php```

Error message can be improved to promt what is required