[edn/dv] Add checks on the alert signal

### Description

Currently the alert vseq only checks whether the alert flag is set in the status register.
Whether the actual alert is raised to the alert handler is not checked.
This is a coverage hole that needs to be filled. We possibly also need to write an alert agent
to allow for the change to happen.

Start with the alert vseq and status-register check described in the issue, then trace how the alert handler receives the actual alert signal. Done means the verification checks that the alert reaches the handler, with an alert agent added if required to trigger the change.

Written by the indexing model from the issue text.