[dv] Check if alert is configured to fatal correctly

From issue #16159 and discussion https://github.com/lowRISC/opentitan/pull/16176#issuecomment-1309336183
We need a better way to check if `is_fatal` parameter is configured correctly, regardless of alert request is sticky or not.

To solve this issue, we can consider adding a DV interface for prim_alert_sender and drive `alert_req` manually to make sure the `IsFatal` is set correctly.

Start with issue #16159 and the discussion in pull request #16176, then inspect the prim_alert_sender verification flow. Focus on how a DV interface could drive alert_req manually and check IsFatal for both sticky and non-sticky alert requests. Done means the check reliably detects incorrect IsFatal configuration regardless of alert behavior.

Written by the indexing model from the issue text.