run-tests: introduce --random for running tests in random error

This is convenient when investigating failures one by one to avoid running the
same fixed tests first every time.

tests/run-tests.py

@@ -198,6 +198,8 b' def parseargs():'
         help="enable Py3k warnings on Python 2.6+")
     parser.add_option('--extra-config-opt', action="append",
                       help='set the given config opt in the test hgrc')
+    parser.add_option('--random', action="store_true",
+                      help='run tests in random order')
 
     for option, (envvar, default) in defaults.items():
         defaults[option] = type(default)(os.environ.get(envvar, default))
@@ -1248,6 +1250,9 b' def main():'
 
     tests = args
 
+    if options.random:
+        random.shuffle(tests)
+
     # Reset some environment variables to well-known values so that
     # the tests produce repeatable output.
     os.environ['LANG'] = os.environ['LC_ALL'] = os.environ['LANGUAGE'] = 'C'