Only use one process when running tests on Windows

tools/testing/test_runner.py

Index: tools/testing/test_runner.py
diff --git a/tools/testing/test_runner.py b/tools/testing/test_runner.py
index 3ef2376f0e763fd9b019e7f90e79e4ea3af02502..00bdf38ebb3c9230df0f28d0ab9bf937fdfd3ff4 100755
--- a/tools/testing/test_runner.py
+++ b/tools/testing/test_runner.py
@@ -225,9 +225,13 @@ class BatchRunner(TestRunner):
 
     # Scale the number of tasks to the nubmer of CPUs on the machine
     # 1:1 is too much of an overload on many machines in batch mode,
-    # so scale the ratio of threads to CPUs back.
+    # so scale the ratio of threads to CPUs back. On Windows running
+    # more than one task is not safe.
     if tasks == testing.USE_DEFAULT_CPUS:
-      tasks = .75 * testing.HOST_CPUS
+      if utils.IsWindows():
+        tasks = 1
+      else:
+        tasks = .75 * testing.HOST_CPUS
 
     # Start threads
     for i in xrange(tasks):