On the alerts side, you could exclude it from SkopeIt and Analytics by explicitly excluding it via the search condition.
That could then be saved and set as a Shared Search.
But that wouldn't exclude it from showing in pre-built reports and dashboards (such as the home page).
To prevent the Alert from firing at all, I would clone the rule(s) that generate the alert and place the new rule immediately ahead of the cloned rule. Then I would add a source constraint of your pentest user and change the action to Allow.