generate_test_keys: add default output file option

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>
diff --git a/tests/scripts/generate_test_keys.py b/tests/scripts/generate_test_keys.py
index 9920933..dbdb960 100755
--- a/tests/scripts/generate_test_keys.py
+++ b/tests/scripts/generate_test_keys.py
@@ -91,14 +91,16 @@
     yield "      {0}, sizeof({0}) }},".format(pub_array_name)
 
 def main() -> None:
+    current_path = os.path.dirname(os.path.realpath(__file__))
+    default_output_path = current_path + "/../src/test_keys.h"
+
     argparser = argparse.ArgumentParser()
-    argparser.add_argument("--output", required=True, help="Output file")
+    argparser.add_argument("--output", help="Output file", default=default_output_path)
     args = argparser.parse_args()
 
     output_file = args.output
     # Remove output file if already existing.
     if os.path.exists(output_file):
-        print("Warning: {} already existing, it will be overwritten.", output_file)
         os.remove(output_file)
 
     output_file = open(output_file, 'at')