Fixes to invalidation logic in icache core agent

Firstly, the pulse shouldn't be zero length (since that wouldn't
actually do anything).

Also, 1 time in 500 is too rare for either invalidations or "long
invalidations", I think, so this patch also increases how often we see
each.

dv/uvm/icache/dv/ibex_icache_core_agent/ibex_icache_core_driver.sv

@@ -143,7 +143,7 @@ class ibex_icache_core_driver
   virtual task automatic invalidate();
     int unsigned num_cycles;
     `DV_CHECK_STD_RANDOMIZE_WITH_FATAL(num_cycles,
-                                       num_cycles dist { 0 :/ 499, [1:20] :/ 1 };)
+                                       num_cycles dist { 1 :/ 10, [2:20] :/ 1 };)
     cfg.vif.invalidate_pulse(num_cycles);
   endtask
 

dv/uvm/icache/dv/ibex_icache_core_agent/ibex_icache_core_if.sv

@@ -84,7 +84,7 @@ interface ibex_icache_core_if (input clk, input rst_n);
   //
   // A one-cycle pulse will start an invalidation, but testing might want a longer pulse (which the
   // cache should support)
-  task automatic invalidate_pulse(int num_cycles);
+  task automatic invalidate_pulse(int unsigned num_cycles);
     driver_cb.invalidate <= 1'b1;
     wait_clks(num_cycles);
     driver_cb.invalidate <= 1'b0;

dv/uvm/icache/dv/ibex_icache_core_agent/ibex_icache_core_req_item.sv

@@ -45,9 +45,9 @@ class ibex_icache_core_req_item extends uvm_sequence_item;
   }
 
   constraint c_invalidate_dist {
-    // Poke the cache invalidate line one time in 500. This takes ages and we don't want to
+    // Poke the cache invalidate line one time in 50. This takes ages and we don't want to
     // accidentally spend most of the test waiting for invalidation.
-    invalidate dist { 0 :/ 499, 1 :/ 1 };
+    invalidate dist { 0 :/ 49, 1 :/ 1 };
   }
 
   constraint c_num_insns_dist {