mavsdk_tests: write output to logs folder

And create folder if it doesn't exist already.

test/mavsdk_tests/mavsdk_test_runner.py

@@ -166,7 +166,7 @@ def main():
 
     parser = argparse.ArgumentParser()
     parser.add_argument("--log-dir",
-                        help="Directory for log files, stdout if not provided")
+                        help="Directory for log files", default="logs")
     parser.add_argument("--speed-factor", default=1,
                         help="How fast to run the simulation")
     parser.add_argument("--iterations", type=int, default=1,
@@ -193,6 +193,8 @@ def main():
     if not is_everything_ready(config):
         sys.exit(1)
 
+    os.makedirs(args.log_dir, exist_ok=True)
+
     run(args, config)