Test verifyTransformation()

[dmorejon]

Currently, verifyTransform() in verify.js is untested since it actually runs run_web_tests.py hardcoded. It would be nice to have this as a path passed in as a flag so we can pass a dummy script to test the function.

Also, we might be able to add support for running the tool (not just testing) from a different directory than web_tests using the --run-web-tests flag. However, this has different behavior with baseline files and output, so more work needs to be done for that support.