Allow conformance test runner to tolerate crashes, and re-enable conformance tests.

python/tox.ini

diff --git a/python/tox.ini b/python/tox.ini
index 301335b..cf8d540 100644
--- a/python/tox.ini
+++ b/python/tox.ini
@@ -15,10 +15,8 @@
     cpp: python setup.py -q build --cpp_implementation --warnings_as_errors
     python: python setup.py -q test -q
     cpp: python setup.py -q test -q --cpp_implementation
-# TODO(xiaofeng): Disabled because the Python testee program crashes on
-# some conformance test inputs.
-# python: python setup.py -q test_conformance
-# cpp: python setup.py -q test_conformance --cpp_implementation
+    python: python setup.py -q test_conformance
+    cpp: python setup.py -q test_conformance --cpp_implementation
 deps =
     # Keep this list of dependencies in sync with setup.py.
     six>=1.9