diagnostics: Skipped test should not be considered as an issue

https://salsa.debian.org/freedombox-team/freedombox/-/blob/main/plinth/modules/diagnostics/__init__.py?ref_type=heads#L295

We are checking if the result is not passed, and anything other than a warning is considered an error. This causes the notification to show that there is an error, when it is just a test that was skipped.