Let test runner rerun failures to test for flakes.

tools/run-tests.py

Index: tools/run-tests.py
diff --git a/tools/run-tests.py b/tools/run-tests.py
index fbbe41641f29fa975920edec2624d06e70134c71..03ad716c67328ddfb4656188cbeba1a68330987d 100755
--- a/tools/run-tests.py
+++ b/tools/run-tests.py
@@ -181,6 +181,13 @@ def BuildOptions():
                     default=False, action="store_true")
   result.add_option("--json-test-results",
                     help="Path to a file for storing json results.")
+  result.add_option("--rerun-failures-count",
+                    help=("Number of times to rerun each failing test case. "
+                          "Very slow tests will be rerun only once."),
+                    default=0, type="int")
+  result.add_option("--rerun-failures-max",
+                    help="Maximum number of failing test cases to rerun.",
+                    default=100, type="int")
   result.add_option("--shard-count",
                     help="Split testsuites into this number of shards",
                     default=1, type="int")
@@ -416,7 +423,9 @@ def Execute(arch, mode, args, options, suites, workspace):
                         options.extra_flags,
                         options.no_i18n,
                         options.random_seed,
-                        options.no_sorting)
+                        options.no_sorting,
+                        options.rerun_failures_count,
+                        options.rerun_failures_max)
 
   # TODO(all): Combine "simulator" and "simulator_run".
   simulator_run = not options.dont_skip_simulator_slow_tests and \