[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 <chencindy@opentitan.org>

hw/ip/prim/dv/prim_alert/tb/prim_alert_tb.sv

diff --git a/hw/ip/prim/dv/prim_alert/tb/prim_alert_tb.sv b/hw/ip/prim/dv/prim_alert/tb/prim_alert_tb.sv
index 9ac75fe..e77bb74 100644
--- a/hw/ip/prim/dv/prim_alert/tb/prim_alert_tb.sv
+++ b/hw/ip/prim/dv/prim_alert/tb/prim_alert_tb.sv
@@ -39,6 +39,9 @@
   // The main concern here is the minimal wait cycles between each handshake.
   localparam int  MinHandshakeWait = 2 + WaitCycle;
 
+  // Clock cycles for alert init handshake to finish.
+  localparam int  WaitAlertInitDone = 20;
+
   typedef enum bit [3:0]{
     AlertSet,
     AlertAckSet,
@@ -203,10 +206,7 @@
     main_clk.apply_reset();
 
     // Wait for initialization sequence to end
-    // This should take no more than 20 cycles
-    // if the sender / receiver clocks are on
-    // the same clock domain.
-    main_clk.wait_clks(20);
+    main_clk.wait_clks(WaitAlertInitDone);
 
     // Sequence 1). Alert request sequence.
     main_clk.wait_clks($urandom_range(0, 10));
@@ -230,6 +230,7 @@
       main_clk.wait_clks(1);
       check_alert_handshake(.exp_ping_value(0));
       main_clk.apply_reset();
+      main_clk.wait_clks(WaitAlertInitDone);
     end
     $display("Alert request sequence finished!");