Tests: add usage note to check-rpc-mappings.py

Author: Mason Simon

test/lint/check-rpc-mappings.py

@@ -90,6 +90,10 @@ def process_mapping(fname):
     return cmds
 
 def main():
+    if len(sys.argv) != 2:
+        print('Usage: {} ROOT-DIR'.format(sys.argv[0]), file=sys.stderr)
+        sys.exit(1)
+
     root = sys.argv[1]
 
     # Get all commands from dispatch tables