[dv/prim_alert] Fix async fatal alert regression error
[dv/prim_alert] Fix async fatal alert regression error

This PR fixes the async fatal alert regression error due to alert init
sequence.
After reset, testbench did not wait long enough for alert init sequence
to finish.

Signed-off-by: Cindy Chen <399ad771a111eac8923348b1e2af093034e5d4f1@opentitan.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan