Do not pregenerate update payloads in parallel.

bin/cros_au_test_harness.py

Index: bin/cros_au_test_harness.py
diff --git a/bin/cros_au_test_harness.py b/bin/cros_au_test_harness.py
index 47d51ad29ec24b5d2cc4925f0b3ad02ccbc39ab5..5e4faf2fe57157b8b31033b064471bb262c7da0d 100755
--- a/bin/cros_au_test_harness.py
+++ b/bin/cros_au_test_harness.py
@@ -911,7 +911,9 @@ def _RunTestsInParallel(parser, options, test_class):
     threads.append(unittest.TextTestRunner().run)
     args.append(test_case)
 
-  results = _RunParallelJobs(options.jobs, threads, args, print_status=False)
+  # TODO(sosa): Set back to options.jobs once parallel generation is
+  # no longer flaky.
+  results = _RunParallelJobs(1, threads, args, print_status=False)
   for test_result in results:
     if not test_result.wasSuccessful():
       Die('Test harness was not successful')