Finally fix on_error_test hang.
Finally fix on_error_test hang.

This is due to recent python version which verifies certificate validity.

R=vadimsh@chromium.org
BUG=695897

Review-Url: https://codereview.chromium.org/2857443002

luci/luci-py,luci/luci-py,luci/luci-py,luci/luci-py