Make mojob.py test succeed on Windows.

Python tests aren't supported because the bindings aren't built on Windows.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/718773002
diff --git a/mojo/tools/test_runner.py b/mojo/tools/test_runner.py
index 28eb3be..ea892af 100755
--- a/mojo/tools/test_runner.py
+++ b/mojo/tools/test_runner.py
@@ -7,6 +7,7 @@
 
 import logging
 import os
+import platform
 import subprocess
 import sys
 
@@ -71,6 +72,9 @@
     else:
       cacheable = True
 
+    if platform.system() == 'Windows':
+      gtest += ".exe"
+
     if successes_cache_file and cacheable:
       _logging.debug("Getting transitive hash for %s ... " % gtest)
       try: