[test] Add random seed stress mode to test runner.

tools/testrunner/local/execution.py

Index: tools/testrunner/local/execution.py
diff --git a/tools/testrunner/local/execution.py b/tools/testrunner/local/execution.py
index 4abf614219fc02ae634b9e1330a425386905430e..dccd52a359976dfcf0386f50fcdfa385cd320399 100644
--- a/tools/testrunner/local/execution.py
+++ b/tools/testrunner/local/execution.py
@@ -277,10 +277,11 @@ class Runner(object):
       d8testflag = ["--test"]
     if utils.IsWindows():
       shell += ".exe"
+    if self.context.random_seed:
+      d8testflag += ["--random-seed=%s" % self.context.random_seed]
     cmd = (self.context.command_prefix +
            [os.path.abspath(os.path.join(self.context.shell_dir, shell))] +
            d8testflag +
-           ["--random-seed=%s" % self.context.random_seed] +
            test.suite.GetFlagsForTestCase(test, self.context) +
            self.context.extra_flags)
     return cmd