Make run-tests.py warn when it's not testing anything

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 514 matching lines @@
   for (arch, mode) in options.arch_and_mode:
     try:
       code = Execute(arch, mode, args, options, suites, workspace)
     except KeyboardInterrupt:
       return 2
     exit_code = exit_code or code
   return exit_code
 
 
 def Execute(arch, mode, args, options, suites, workspace):
-  print(">>> Running tests for %s.%s" % (arch, mode))
-
   shell_dir = options.shell_dir
   if not shell_dir:
     if options.buildbot:
       # TODO(machenbach): Get rid of different output folder location on
       # buildbot. Currently this is capitalized Release and Debug.
       shell_dir = os.path.join(workspace, options.outdir, mode)
       mode = BuildbotToV8Mode(mode)
     else:
       shell_dir = os.path.join(
           workspace,
@@ skipping 97 matching lines @@
       ]
     else:
       s.tests = variant_tests
 
     s.tests = ShardTests(s.tests, options.shard_count, options.shard_run)
     num_tests += len(s.tests)
 
   if options.cat:
     return 0  # We're done here.
 
+  if num_tests == 0:
+    print("No tests selected for running!")
+    if len(all_tests) > 0:
+      verbose.PrintReport(all_tests)
+    return 0
+
+  print(">>> Running tests for %s.%s" % (arch, mode))
+
   if options.report:
     verbose.PrintReport(all_tests)
 
   # Run the tests, either locally or distributed on the network.
   start_time = time.time()
   progress_indicator = progress.IndicatorNotifier()
   progress_indicator.Register(progress.PROGRESS_INDICATORS[options.progress]())
   if options.junitout:
     progress_indicator.Register(progress.JUnitTestProgressIndicator(
         options.junitout, options.junittestsuite))
@@ skipping 30 matching lines @@
   exit_code = runner.Run(options.j)
   overall_duration = time.time() - start_time
 
   if options.time:
     verbose.PrintTestDurations(suites, overall_duration)
   return exit_code
 
 
 if __name__ == "__main__":
   sys.exit(Main())