Don't pass --iterations to list_tests.py if not overriding it

This fixes a bug introduced by 14c400c, where I sort of half made this
change but didn't follow through the whole way!

dv/uvm/core_ibex/Makefile

@@ -54,9 +54,9 @@ TEST                := all
 TESTLIST            := riscv_dv_extension/testlist.yaml
 # Verbose logging
 VERBOSE             :=
-# Number of iterations for each test, assign a non-zero value to override the
+# Number of iterations for each test, assign a non-empty value to override the
 # iteration count in the test list
-ITERATIONS          := 0
+ITERATIONS          :=
 # Generator timeout limit in seconds
 TIMEOUT             := 1800
 # Privileged CSR YAML description file
@@ -184,7 +184,7 @@ tests-and-seeds := \
   $(shell ./list_tests.py \
             --start_seed $(SEED) \
             --test "$(TEST)" \
-            --iterations $(ITERATIONS) \
+            $(if $(ITERATIONS),--iterations $(ITERATIONS),) \
             --ibex-config $(IBEX_CONFIG))
 
 # Define a variable that contains the output directories for all the

dv/uvm/core_ibex/list_tests.py

@@ -142,12 +142,12 @@ def main() -> int:
     parser = argparse.ArgumentParser()
     parser.add_argument('--start_seed', type=int, default=1)
     parser.add_argument('--test', required=True)
-    parser.add_argument('--iterations', type=int, default=0)
+    parser.add_argument('--iterations', type=int)
     parser.add_argument('--ibex-config', required=True)
 
     args = parser.parse_args()
-    if args.iterations < 0:
-        raise RuntimeError('Bad --iterations argument: must be non-negative')
+    if args.iterations is not None and args.iterations <= 0:
+        raise RuntimeError('Bad --iterations argument: must be positive')
     if args.start_seed < 0:
         raise RuntimeError('Bad --start_seed argument: must be non-negative')