Prepare python code for reftests.

-s/-expected.html/-expected.sky/
-s/-expected-mismatch.html/-expected-mismatch.sky/
-In single_test_runner.py, get rid of the image hash checking
for reftests. It doesn't provide any value and just makes
getting reftests working more complicated.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/802573002