Fix a broken test in dimension_mapping_test
Fix a broken test in dimension_mapping_test

BUG=
R=maruel@chromium.org

Review URL: https://codereview.appspot.com/34640046

madecoste/swarming,madecoste/swarming,luci/luci-py,luci/luci-py,pombreda/swarming,pombreda/swarming,pombreda/swarming,madecoste/swarming,madecoste/swarming,luci/luci-py,luci/luci-py,pombreda/swarming