[test] Make the meaning of some options clearer in --help.

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 272 matching lines @@
   result.add_option("--no-sorting", "--nosorting",
                     help="Don't sort tests according to duration of last run.",
                     default=False, dest="no_sorting", action="store_true")
   result.add_option("--no-stress", "--nostress",
                     help="Don't run crankshaft --always-opt --stress-op test",
                     default=False, dest="no_stress", action="store_true")
   result.add_option("--no-variants", "--novariants",
                     help="Don't run any testing variants",
                     default=False, dest="no_variants", action="store_true")
   result.add_option("--variants",
-                    help="Comma-separated list of testing variants: %s" % VARIANTS)
+                    help="Comma-separated list of testing variants;"
+                    " default: \"%s\"" % ",".join(VARIANTS))
   result.add_option("--exhaustive-variants",
                     default=False, action="store_true",
-                    help="Use exhaustive set of default variants.")
+                    help="Use exhaustive set of default variants:"
+                    " \"%s\"" % ",".join(EXHAUSTIVE_VARIANTS))
   result.add_option("--outdir", help="Base directory with compile output",
                     default="out")
   result.add_option("--predictable",
                     help="Compare output of several reruns of each test",
                     default=False, action="store_true")
   result.add_option("-p", "--progress",
                     help=("The style of progress indicator"
                           " (verbose, dots, color, mono)"),
                     choices=progress.PROGRESS_INDICATORS.keys(), default="mono")
   result.add_option("--quickcheck", default=False, action="store_true",
@@ skipping 563 matching lines @@
         "--coverage-dir=%s" % options.sancov_dir])
     except:
       print >> sys.stderr, "Error: Merging sancov files failed."
       exit_code = 1
 
   return exit_code
 
 
 if __name__ == "__main__":
   sys.exit(Main())