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

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index bd8330f5ae0284a903a2731c57d59c6661972810..dc6ff9f7d63c5597afa2b598e388b1e071f163ee 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -301,14 +301,24 @@ def BuildOptions():
   result.add_option("--junittestsuite",
                     help="The testsuite name in the JUnit output file",
                     default="v8tests")
-  result.add_option("--random-seed", default=0, dest="random_seed",
+  result.add_option("--random-seed", default=0, dest="random_seed", type="int",
                     help="Default seed for initializing random generator")
+  result.add_option("--random-seed-stress-count", default=1, type="int",
+                    dest="random_seed_stress_count",
+                    help="Number of runs with different random seeds")
   result.add_option("--msan",
                     help="Regard test expectations for MSAN",
                     default=False, action="store_true")
   return result
 
 
+def RandomSeed():
+  seed = 0
+  while not seed:
+    seed = random.SystemRandom().randint(-2147483648, 2147483647)
+  return seed
+
+
 def ProcessOptions(options):
   global VARIANT_FLAGS
   global VARIANTS
@@ -373,8 +383,8 @@ def ProcessOptions(options):
   if options.j == 0:
     options.j = multiprocessing.cpu_count()
 
-  while options.random_seed == 0:
-    options.random_seed = random.SystemRandom().randint(-2147483648, 2147483647)
+  if options.random_seed_stress_count <= 1 and options.random_seed == 0:
+    options.random_seed = RandomSeed()
 
   def excl(*args):
     """Returns true if zero or one of multiple arguments are true."""
@@ -596,9 +606,28 @@ def Execute(arch, mode, args, options, suites, workspace):
       verbose.PrintTestSource(s.tests)
       continue
     variant_flags = [VARIANT_FLAGS[var] for var in VARIANTS]
-    s.tests = [ t.CopyAddingFlags(v)
-                for t in s.tests
-                for v in s.VariantFlags(t, variant_flags) ]
+    variant_tests = [ t.CopyAddingFlags(v)
+                      for t in s.tests
+                      for v in s.VariantFlags(t, variant_flags) ]
+
+    if options.random_seed_stress_count > 1:
+      # Duplicate test for random seed stress mode.
+      def iter_seed_flags():
+        for i in range(0, options.random_seed_stress_count):
+          # Use given random seed for all runs (set by default in execution.py)
+          # or a new random seed if none is specified.
+          if options.random_seed:
+            yield []
+          else:
+            yield ["--random-seed=%d" % RandomSeed()]
+      s.tests = [
+        t.CopyAddingFlags(v)
+        for t in variant_tests
+        for v in iter_seed_flags()
+      ]
+    else:
+      s.tests = variant_tests
+
     s.tests = ShardTests(s.tests, options.shard_count, options.shard_run)
     num_tests += len(s.tests)