run-tests.py: warn when no tests were run

tools/run-tests.py

 #!/usr/bin/env python
 #
 # Copyright 2012 the V8 project authors. All rights reserved.
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 #       notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 679 matching lines @@
     runner = network_execution.NetworkedRunner(suites, progress_indicator,
                                                ctx, peers, workspace)
   else:
     runner = execution.Runner(suites, progress_indicator, ctx)
 
   exit_code = runner.Run(options.j)
   overall_duration = time.time() - start_time
 
   if options.time:
     verbose.PrintTestDurations(suites, overall_duration)
+
+  if num_tests == 0:
+    print("Warning: no tests were run!")
+
   return exit_code
 
 
 if __name__ == "__main__":
   sys.exit(Main())