[testrunner] preventing a StopIteration exception when running tests

The next built-in function throws a "StopIteration" when iterator is exhausted
which will crash and stop the test runner. Asking it to return a "None" instead will
fix the issue.

Change-Id: I3ca4ff8f28f359798c9faf6f1ffbbaa6bb95f156
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1524719
Commit-Queue: Milad Farazmand <miladfar@ca.ibm.com>
Reviewed-by: Tamer Tas <tmrts@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60294}

tools/testrunner/local/testsuite.py

@@ -268,7 +268,7 @@ class TestSuite(object):
 
   def __initialize_test_count_estimation(self):
     # Retrieves a single test to initialize the test generator.
-    next(iter(self.ListTests()))
+    next(iter(self.ListTests()), None)
 
   def __calculate_test_count(self):
     self.__initialize_test_count_estimation()